Kibitzer
A kibitzer is a spectator who watches a game and comments on it, usually without being asked.
The simple explanation
At a poker table, a kibitzer is a person who stands or sits near the game and talks about the hands. Players often find this annoying, especially when the kibitzer sees someone's cards.
A closer look
Rooms have rules to protect games from kibitzers. A person who sees a player's cards should not speak about them, because it can give information to an opponent or even help cheating. Many cardrooms limit where spectators can stand. In friendly home games, a kibitzer is often welcome, but a quiet one is better.
A practical example
A friend of one of the players stands behind his chair and comments on every pot, so the dealer asks him to step back.
Full explanation
A kibitzer is a spectator who watches a game and comments on it, usually without being asked.
A kibitzer is a spectator who comments on the game.
He usually gives opinions that nobody asked for.
A spectator should not reveal anybody’s cards.
Rooms may restrict where spectators stand.