These are special moves of chess and how to use them.
Castling is a strategic move that moves the King and Rook in one move. It can get the king out of tight positions and free the rook to move around the board. You can only castle when the King is not in check. Also, the Rook and King that are castling can not have moved once during the game. Another requirement to castle is that there can not be any pieces between the Rook and King. See the pictures below for examples.
Castling on the king side:
Castling on the queen side:
En Passant is a special move that allows you to take a pawn when the pawn is not diagonal to your pawn. If your opponent moves a pawn 2 spaces on the first move, you can still take the pawn as if it moved only one space. The picture below shows a more clear example.