The object of the game is to put the opponent's king in check so securely that escape is impossible. This is called checkmate.
In this illustration, Black coordinates a king and two rooks to checkmate the white king. The black rook from c6 captures the white rook at c3, putting the white king in check. Since the checking piece (rook at c3) is right up against the checked king, it cannot be blocked. The checking piece cannot be captured because it is out of the white knight's range. The white king can not capture the checking piece because to do so would be moving into check. The king is in check and cannot escape. Checkmate has been accomplished. Black wins this match.