BASICS
The most basic part of gameplay in a TTRPG is the communication between the Gamemaster (GM) and the players. The Gamemaster describes the environment, including other characters, places, and items, then players choose to interact with the environment through their Player Character. Then the Gamemaster uses the rule system of the game to determine the outcome of that interaction, and then describes the outcome. Then the Players interact again, and the cycle repeats.
DETERMINATION OF OUTCOMES
The outcome of a given action is determined by the Gamemaster. If there is no doubt as to the outcome of an action, the Gamemaster will simply describe the outcome. If the action being taken could succeed or fail, then the determination of the outcome is decided by rolling dice and adding modifiers based on the situation and your character's items and abilities.