Original: In conjunction with a rook, the king may make a special move called castling, in which the king moves two squares toward one of its rooks and then the rook is placed on the other side of the king.
Simplified: With a rook, the king may make a special move called 'castling'. This is when the king moves two squares toward one of its rooks and then the rook is placed on the other side of the king.