The Roll
If you've been around tabletop role-playing games, you'll know the core of the game is rolling dice — and if you're new to them, welcome to the most exhilarating part of the game.
A Game Master (GM) can choose how to handle each situation, but the most common way to resolve a scenario is with the roll of the dice. Often the player declares what they'd like to do, the GM decides how difficult that action will be and chooses a Difficulty Class (DC) — a number the player has to beat, which they usually keep secret until after the result of the roll. Next, the player rolls the dice and adds any bonuses to the result, and finally the GM narrates the outcome.
Four Outcomes
There are four ways the roll can result — critical success, success, failure, or critical failure. Start with the total, roll plus bonuses, against the DC: beat it by 10 or more and it's a critical success; meet or beat it and it's a success; miss it and it's a failure; miss it by 10 or more and it's a critical failure.
Then the face of the die nudges the result one step. A natural 20 — the highest the die can show — bumps your outcome up one degree, and a natural 1 bumps it down one. So a "Nat 20" turns a success into a critical success, or rescues a near-miss into a plain success; a "Nat 1" drops a hit to a miss, or a miss into a critical failure. The die doesn't hand you a critical by itself — it moves you one step from wherever your total already landed.
At the Table
Here are a couple of examples of what that might look like: