What "kingside castling" means in chess

Kingside castling is a move where the king and rook on the player's king's side are moved simultaneously. The king is moved two squares towards the rook and the rook is moved to the square over which the king passed. This move can only be done if there are no pieces between the king and rook and if the king is not in check, has not moved before, and will not be in check or pass through a square under attack. Kingside castling is denoted by the symbol "0-0" or "O-O" in algebraic notation.