The Dungeon Master or DM (Also referred to as Game Master, Game Moderator, GM, etc.) is the organizer of a scenario and is in charge of everything that transpires within a game, save for the actions of the players.
They will explain to the players everything that they are seeing, hearing and otherwise experiencing, so as to help the player to make an informed decision about their next move. Unlike in tabletop roleplaying games, you cannot see the other players rolling the dice, so to avoid exploitation and to keep it fair, the DM is in charge of all of the dice rolls and other calculations when players are engaged in combat or other activity.
The DM should follow the rules of this wiki if they want to make sure they run a fair game, though they can expand upon them if they wish.