Castling is a special move where the king moves two squares toward a rook and that rook jumps to the king's other side. It is the only move that shifts two pieces at once, and it both tucks the king to safety and activates the rook. Short castling on the kingside is quicker; long castling on the queenside takes the king a bit further from the edge. You may castle only if neither the king nor that rook has moved, the squares between them are empty, and the king is not in check, does not pass through check, and does not land in check.