What is chess castling?

Castling in Chess is a special move where you move two pieces at once (your king and a rook). To castle the following conditions must be met: 1) the king and the rook have not been moved, 2) the king is not in check, 3) the king will not move through or end up in check, and 4) there are no pieces between the king and rook. To castle, move the king two squares toward the rook, then move the rook to the square on the other side of the king and adjacent to it like below: K _ _ R (before castling) _ R K _ (after castling) R _ _ _ K (before castling) _ _ K R _ (after castling)