When a character attempts an action, the player rolls a 20-sided die (D20).
The result of the die is then combined with the appropriate character attribute modifier to obtain the final result.
This mechanic is called a skill check or simply a check.
Example:
D20 + Strength (STR) for a physical action, D20 + Intelligence (INT) for an intellectual action.
This result is compared to a success threshold determined by the Game Master (GM), which the GM may choose to announce in advance or not.