Normally, a creature that can perceive you using at least one of its five senses (hearing, sight, smell, taste, or touch) automatically knows your location. A creature that can't perceive you doesn't know where you are. When you use the Hide action, you attempt to conceal your location by making it harder for creatures to perceive you.

Make an Agility test and record the result. Until you stop hiding, the GM secretly opposes that result by the passive Intellect test (see Passive Tests) of each other creature you encounter during the hide. You are considered to be hidden from every creature you succeed against.

Being Hidden: When you are hidden from a creature, it can perceive you only if it can perceive you clearly. In other words, as long as you stay quiet, keep out of plain view, don't stink, and avoid physical contact, the creature can't locate you (but see below).

The GM determines what "perceive you clearly" means, based on the circumstances and the nature of the environment. The easiest way to avoid being seen clearly is to hide in partial or total cover, or in an area that is lightly or heavily obscured (see Vision and Light). The GM can also rule that a distracted creature fails to see you clearly.

Breaking Stealth: You stop hiding if you are incapacitated, or if you are located by a hostile creature. When you make an attack, you stop hiding once the attack has been resolved. Alternatively, you can stop hiding whenever you like (no action required).

No Guarantees: A creature can use the Search action to oppose your recorded Agility result with an active test. An observant creature can simply deduce your location, if its reasoning is founded on accurate information. Even with no information, a creature might make a lucky guess, or come upon you by accident!