Check is when a king is directly attacked by an enemy piece and must respond immediately. You have three ways to get out of check: move the king to a safe square, block the attack with another piece, or capture the checking piece. You cannot ignore a check or make any other move while in it, and you may never leave your own king in check. If none of the three escapes is possible, it is checkmate and the game is lost. Giving check can be useful, but random checks that the opponent simply blocks often just waste time and help them develop.