Rules :: When am I in check?
In chess, the most valuable piece in the King, if it is threatened you must get it somewhere safe (not attacked by any enemy piece). If you have no way of getting it out of danger, then it is checkmate and you lost the game. Here are a few examples of the King being in check.

The King is threatened by the Rook here, he is in check.

There are 3 possible ways to get out of check: