Check
The king is attacked
A piece that attacks the king gives check. You must deal with it at once.
Three ways out
Move the king, capture the attacker, or put a piece in the way. Try each one.
The king may never move into check, and you may never make a move that leaves your own king in check. A move like that is not allowed, so the board will not let you make it.
When a king is in check and none of the three ways works, it is checkmate and the game is over. That is Unit 2.