Bonus to pass tests of skill to:
Hide or sneak. Avoid detection. Conceal light weapons. Do something sneaky or sneakily. Steal stuff. Pick pockets. Pick locks. Case an area. Detect, disarm, disable, bypass, or create traps. Sabotage. Disguise. Create a forgery. Perform sleights of hand. Trip or tumble.

This bonus to tests of skill is a training bonus.
If you have more than one training bonus which applies to the same roll, use the best one.

Trip / Tumble
= d20 + Health + Strength or Agility or Roguery
vs. d20 + Health + Strength or Agility

Hiding Rules:
The short and simple rule is: You may hide if no enemy can see you clearly.

Hide
= d20 + Health + Roguery
vs. d20 + Health, Social, or Academics + Seeking

Some abilities (Losing Them, Shadow Arts) change how the hiding rules work for you.

Concealment Required:
You must be at least partially concealed from all enemies to hide.
If you are concealed from all enemies, you may hide normally.
If you are not concealed from all enemies, but you are at least partially concealed from all enemies, you may hide, but have disadvantage on your roll to hide.
If you ever stop being at least partially concealed from all enemies (any enemy can clearly see you), you immediately stop being hidden.

Hide:
You hide as an action by rolling a contest of Roguery against a test of Seeking from each enemy. If your test of Roguery beats their highest test of Seeking, you become hidden.

Only those enemies which roll higher than you are aware of where you are, however, in combat they can readily point you out to their allies as a free action, making them all aware of your location. Meaning after each creature takes their first turn in combat, you will normally be either hidden from all enemies or not hidden from any of them.
However, in certain situations such as ambushes which are not detected from afar, you may be considered hidden from those enemies which rolled lower than your test of Roguery, and may be able to ambush them before they can respond to the alarm (depending upon how initiative goes), but you cannot ambush those enemies that did detect you.
Therefore, technically, you can be hidden from some creatures and not others, but once a fight breaks out, this is not normally the case.

Hidden:
While you are hidden:
Rolls to hit you have disadvantage.
Enemies are off guard against you (you have advantage on rolls to hit), but attacking will cause you to stop being hidden.
Health (reflex) saves against your abilities have disadvantage, but casting a spell which is not subtle will cause you to stop being hidden.
You have advantage on initiative rolls.
Enemies do not know exactly where you are.
They still know where they last saw you. They may still attack where they think you are. They automatically miss if you are not there (and have disadvantage if you are there). If they do guess correctly and hit you with a melee attack, you stop being hidden.

Breaking Stealth (Becoming Unhidden):
Once you are hidden, you remain hidden until you attack, cast a spell, throw an item, dash, make any noise louder than a whisper, ungently touch or are touched by an enemy (trip, push, pull, are hit by a melee attack, or as one potential outcome of failing a test of Roguery), or if you stop being at least partially concealed (are clearly seen), or are found.
You may have to roll again to remain hidden as part of an extended test.
You may also need to roll against newly arriving or encountered enemies to remain hidden from them.

To elaborate upon losing concealment, if at any point, you stop being at least partially concealed from all enemies, meaning any enemy can clearly see you, you immediately stop being hidden.
(ex: If you are hiding behind a pillar and they go around it and get a line of sight on you, you stop being hidden (unless you have another source of concealment).
ex: If you walk into an area where they can plainly see you, such as a brightly lit area, you stop being hidden.)

As an action, a creature which has reason to do so (such as a patrol conducting a cursory inspection, or a creature which heard something or has other cause to investigate) may seek out where you are hidden by rolling a contest of seeking against a test of roguery from you. If they win, they find you and you stop being hidden.
Certain abilities also grant extra opportunities to seek.

When you take an action which causes you to stop being hidden, you catch the enemy off guard and have advantage on all rolls to hit with that action (even if you are not concealed).

Triggers Unaffected:
Acting while hidden does not, by itself, prevent your target from using reactions.
However, ambushing an enemy will.

Ambush:
An action is an ambush when you act against an enemy while you are hidden,
you have never been unhidden during that combat encounter,
and the enemy has not yet taken its first turn of that combat encounter.
Enemies cannot use reactions against ambushes unless they are effortless.

The first time any enemy detects you, all enemies are typically put on guard and ready to react in short order. Once a fight breaks out, enemies are on guard once they get their first turn.
There is a brief window of opportunity before an enemy you successfully hid from takes their first turn in which you may normally ambush them before they have a chance to raise their guard, even if another enemy detected you and raised an alarm.
(You cannot ambush if you start an encounter unhidden, or become unhidden at any time during an encounter for any reason)

Concealed:
You are concealed if you are behind a solid object which completely covers you, in an area of complete darkness, invisible, or if an enemy is blind or otherwise unable to see you at all, or sense you using a sight-like ability they possess such as shapesense, heatsense, tremorsense, or lifesense. Being concealed is not the same as being hidden, you may be concealed without being hidden, which means enemies know where you are (as they can hear or sense you), may target you, but cannot see you.
Rolls to hit you from creatures you are concealed from have disadvantage.
Rolls to hit creatures you are concealed from have advantage because they are off guard against you.
If you are concealed from all enemies, you may hide.

Partially Concealed:
You are partially concealed if you are hiding behind or in an object which only partially hides you, such as thin fog, thin smoke, or in dim light or shadow, or in complete darkness against a creature with darkvision. If you are at least partially concealed from all enemies, you may hide with disadvantage.
An ally may spend an action to try to distract a creature, which may afford other creatures partial concealment from it until the start of that ally’s next turn, or if a creature is particularly focused elsewhere, the Headmaster may award you partial concealment from it by virtue of distraction. This is an exceptional case, in a normal fight, enemies will not naturally be distracted enough for you to hide.

Not Concealed:
You are not concealed if you are standing where an enemy can clearly see you or detect you using a sight-like ability they possess such as shapesense, heatsense, tremorsense, or lifesense. If you are standing in the open in bright light and an enemy is looking at you, you are not concealed. If you are not at least partially concealed from all enemies, you cannot hide. Enemies will cooperate and point you out to each other, so if any enemy can see you clearly, they usually all know where you are. In combat, enemies are aware of their surroundings, and typically, exiting concealment or partial concealment, even momentarily, will cause you to no longer be concealed and no longer hidden, unless an enemy is particularly distracted.
Outside of combat, while vigilant enemies such as a guard posted on watch may be as difficult to hide from as in combat, you may be able to slip past distracted enemies or enemies that are not paying attention, such as a table of enemies playing cards, by using their distraction to conceal your presence.

The Headmaster determines how concealed you are, and under what conditions hiding under a suspiciously lumpy blanket qualifies as concealment, partial concealment, or a paper-thin disguise.