GM — Game Master
The person who runs the game: describes the world, plays every monster and extra character, and makes the final rulings.
The Game Master (GM) runs the game for everyone else. They set up the adventure, describe what the characters see, play every monster and every character no player controls, and decide anything the rules do not cover. In Game Master the app does the bookkeeping and the math; the GM can still change any result afterwards, but play never waits for their approval on routine things.