Questions relating to check
Check is the term for an attack against a King (see king). This attack is defined as a threat by an opposing piece to capture the King on the next move. As the King is the most important piece, the threat must immediately be removed; other moves are illegal.
When the King is put into check, the attacked player must immediately, in the following move,
move the King out of check,
interpose a piece between the King and the attacking piece (if there is only one - in the case of a double check, this option is not possible), or
capture the attacking piece (again, if there is only one).
If none of these options is available, the King is checkmated (see checkmate), and the game is over.