Because TTRPGs are group-oriented games, it's helpful for everyone in a gaming group to be able to communicate well to have the most fun possible! These resources can help.
The Game Master (GM) is the player "running" the game, guiding the rest of the players through the story and world. This doesn't mean that they're "in charge" necessarily, it's more like they're the narrator for the game, keeping the players on track, playing any non-player characters (NPCs) or monsters, and crafting the world.
The GM has many different names for different systems, for example, in D&D they're called a Dungeon Master, or DM.