If a game continues 50 moves without a Pawn move or a capture, the game is drawn. That means 50 moves for White and 50 moves for Black.
The diagram shows an endgame of Rook and Bishop vs. Rook. This is a theoretical endgame known to be drawn if Black plays correctly. In practice it's a difficult endgame for the defense to conduct and there are many ways to go wrong. If Black can survive for 50 moves, the game is a draw.
Computer analysis of theoretical endgames has discovered positions where the stronger side needs more than 50 moves to win. For those exceptional positions the 50 move rule has been altered to allow sufficient time to win.
