If you've ever played chess, you will be aware that a player usually moves only a single piece. We say "usually because there is one time in chess when you can move two pieces in one move. Castling is that move.
Castling is a special move to protect the king on the board and activate the rook. During this move, the king moves two squares to the left or right and the rook moves directly to the other side of the king. Keep in mind that castling can be done only if neither the king nor the rook have made a move, and there are no pieces in the way. Castling cannot be done when in check, into check or through check.
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