The Conversation
A roleplaying game is a conversation between the GM and the players, punctuated by dice rolls to inject uncertainty and surprising turns.
The GM presents the fictional situation in which the player characters find themselves. The players determine the actions of their characters in response to the situation. The GM and the players together judge how the game systems are engaged. The outcomes of the mechanics then change the situation, leading into a new phase of the conversation (new situations, new actions, new judgments, new rolls) creating an ongoing fiction and building “the story” of the game, organically, from a series of discrete moments.
No one is in charge of the story. The story is what happens as a result of the situation presented by the GM, the actions the characters take, the outcomes of the mechanics, and the consequences that result. The story emerges from the unpredictable collision of all of these elements. You play to find out what the story will be.