What is castling in chess?

Castling is a unique move that can be used to protect the king and develop a rook at the same time. It is the only move in chess where two pieces can move at the same time. During castling, the king moves two steps to its right or left and the rook moves across the king and goes to the position next to the king. Castling can be done only when the following two conditions are satisfied. First, there must not be any other pieces in between the king and the rook. Second, the king and the rook must not have been moved before.