Any time a creature attempts to do something with a chance of failure, they roll a “check”. A check is a d20 roll against a target number determined by the circumstances of the check or by the GM. Checks include attack rolls, perception checks, saving throws, skill checks, and flat checks.
Attack Rolls require the attacker to roll a check against the target’s AC (armor class). On a hit, the attacker deals damage. Every creature and many objects have an AC which measures how difficult they are to damage with strikes.
Perception checks are frequently used with the Seek action to locate creatures or objects, such as a goblin hiding behind a barrel or a trap hidden in a door handle. They’re also commonly used when rolling initiative. Unlike many other RPGs, every character is proficient with Perception and advances automatically.
Saving throws are used to resist or avoid dangerous effects like fire breath, disease, and fear. There are three types of saving throws: Fortitude to physically resist thing affecting your body, reflex to avoid dangerous physical effects, and will to resist mental effects.
Skill checks are rolled when using a skill to accomplish a task. Your character will be trained in a variety of skills, and each skill provides certain actions and activities.
Success, Failure, Degree of Success
Any check can result in one of four degrees of success: critical failure, failure, success, and critical success. Not all checks will have a meaningful difference between critical failure and failure or between success and critical success, but many do.
For example: When making a Strike (the basic action where a creature tries to attack and deal damage), a critical failure or a failure mean that the attack missed and does not deal damage. A success deals damage normally, while a critical success (often referred to as a “critical hit”) deals double damage.
A check which meets or exceeds the target number (AC or DC) succeeds. A check which exceeds the target number by 10 or more (subtract the result from the AC or DC, and if the result is 10 or more) is a critical success. A check which fails by 10 or more is a critical failure.
A natural 1 (a 1 on the face of the rolled d20) reduces your degree of success by 1 (example: from failure to critical failure, though in extreme cases a 1 might not otherwise result in failure), while a natural 20 increases your degree of success by 1 (example: success to critical success).