A king is in check when it is under attack by at least one enemy piece.
It is illegal to make a move that places or leaves one's king in check.
In informal games, it is customary to announce "check" when making a move that puts the opponent's king in check (in formal competitions check is rarely announced).
The possible ways to get out of check are:
1. Move the king to a square where it is not in check.
2. Capture the checking piece (possibly with the king).
3. Block the check by placing a piece between the king and the opponent's threatening piece
If it is not possible to get out of check, the king is checkmated and the game is over.