As you play your character, you will describe things that you want to do. The Game Master’s job is to help determine what happens as a result of your actions. In this game, all of these situations are resolved through a single mechanic, called a check: you roll 1d20, add modifiers based on your competency or the situation, and announce the check result. Usually, the Game Master will compare this check result to a difficulty number based on the situation and the inherent difficulty of what you’re trying to accomplish. In many common situations, the process for determining the check modifier and difficulty have been worked out in detail, but always remember that you and the GM can decide to arbitrate situations that haven’t been specifically spelled out in the rules, or modify the check or difficulty of well-established rules based on special circumstances.
If your check result is equal to or greater than the difficulty, or if you roll a natural 20, you succeed. If you roll a natural 20 and your check result is greater than the difficulty, you achieve a critical success.