One of the least known chess moves is en passant. It is a standard rule of chess, not just a tournament rule, and it has been part of the game since the 15th century. Prior to that time, pawns could not do the double move as their first move from the opening rank. When this rule was put in place, en passant was also introduced as a counter.
In short, en passant allows a pawn to capture a pawn which has passed it with a double move.
By Jasampler de la wikipedia en español - Own work, CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=1344534
Now here are the rules:
- This can only be done with a pawn, if there was a bishop at b4 instead of a pawn, it would not be able to capture the passing pawn.
- It must be done on the turn immediately following the double move, if you make another move, you cannot come back later and use en passant to capture the pawn.
- In that regard, black must have a pawn on the 4 rank, or white must have a pawn on the 5 rank for en passant to be possible.
- The capturing pawn moves diagonally for the capture, to the passed square, it does not move to the side.
- En passant can only be used when a pawn has done a double move, if the white pawn in the first picture had been at a3 and moved to a4, en passant would not have been available to black.
- The capture can only be done on the passed-through square, not the originating square. If the black pawn were at b3 it could not use en passant to capture the white pawn at a2 after it moved.
En passant rarely comes up in games, and that is why it is one of the least known rules of chess. When it does come up, though it can be game-changing.
Hope this was helpful and improves your game.