Game show hosts are people who host a show where the contestant must do certain things to win a prize. They usually introduce the game, welcome the contestant, ask the questions that the contestant must answer and tell them if they're right or wrong. Not all game show hosts do this as not all game shows ask the player to answer questions. Sometimes they serve as commentators, commenting the action. What they all do though is try to be funny, telling jokes that fall flat. They also ask stupid questions to the contestant like "how old are you?" or "what's your name?" and try to get the crowd excited.
In video games, game show hosts are usually found in party games, presenting the questions/rounds and setting the atmosphere. The host will always have a smile on his face even when the player loses: His job is just to keep the show exciting and on track.