prompter: I have heard people talking about the chess move "en passant", but I don't know what it is and how it works. Could you explain it to me?

assistant: The en passant rule may be easiest to understand if you first understand what it was designed prevent.

In chess a pawn can be moved two squares in one go if it is the first move of that pawn. This rule was made in order to speed up the early game. However, it may sometimes happen in late game that a pawn that has not yet been moved may be able to move entirely past the opponent's advanced pawns, by moving two squares and entirely skipping a square where the opponent would have been able to take that pawn. The en passant rule was introduced in order to prevent this, because moving two squares would otherwise have provided a powerful method for creating passed pawns and would thus have changed the game much more than was originally intended.

Let's look at a situation like this on the board:

    8 _ _ _ _ k _ _ _
    7 _ _ _ _ p _ _ _
    6 _ _ _ _ _ _ _ _
    5 _ _ _ _ _ P _ _
    4 _ _ _ _ _ _ _ _
    3 _ _ _ _ _ _ _ _
    2 _ _ _ _ _ _ _ _
    1 _ _ _ _ K _ _ _

      a b c d e f g h

Note that due to my graphical limitations I use `k` and `p` for your opponent's king and pawn, and `K` and `P`, for your king and pawn.
Here, if your opponent moves their pawn two squares (e7e5 in algebraic notation), it will have passed the square where your pawn could have taken it (i.e. square e6), had it only been moved by one square:

    8 _ _ _ _ k _ _ _
    7 _ _ _ _ _ _ _ _
    6 _ _ _ _ _ _ _ _
    5 _ _ _ _ p P _ _
    4 _ _ _ _ _ _ _ _
    3 _ _ _ _ _ _ _ _
    2 _ _ _ _ _ _ _ _
    1 _ _ _ _ K _ _ _

      a b c d e f g h

*The en passant rule allows you to take your opponent's pawn that has been moved two squares, with your own pawn, by moving it diagonally to the square your opponent's pawn has moved through on their previous turn.* This is called "en passant." In algebraic notation we may note this by fxe6 e.p., and the resulting position is:

    8 _ _ _ _ k _ _ _
    7 _ _ _ _ _ _ _ _
    6 _ _ _ _ P _ _ _
    5 _ _ _ _ _ _ _ _
    4 _ _ _ _ _ _ _ _
    3 _ _ _ _ _ _ _ _
    2 _ _ _ _ _ _ _ _
    1 _ _ _ _ K _ _ _

      a b c d e f g h

En passant is a common point of confusion, so don't be afraid to ask if you need more clarification! I'll be only too happy to help.

assistant: En passant is a special pawn capturing move. In chess, a pawn can vertically advance to a vacant square ahead, and vertically advance two squares the first time it moves. It can usually only capture a piece that is diagonally in front of them on an adjacent square and move to that square to replace it. An en passant capture can happen under the following requirements:
1. The captured pawn has moved two squares in one move, landing right next to the capturing pawn.
2. The capturing pawn has advanced exactly three ranks.
3. The capture must be performed on the turn immediately after the pawn being captured moves. 

In an en passant capture, the capturing pawn moves diagonally to the square that the captured pawn has skipped in its two-square-advance and captures it. For example, if a black pawn moves from E7 to E5, a white pawn on D5 can capture it by moving to E6. It is the only capture in chess where the capturing piece does not replace the captured piece on the same square.

assistant: “En passant” is a special method of capturing in chess that occurs when a pawn captures a horizontally adjacent enemy pawn that has just made an initial two-square advance. The capturing pawn moves to the square that the enemy pawn passed over, as if the enemy pawn had advanced only one square.

This move was invented in the 15th century to make chess more interesting and faster. In the olden times, pawns could move just one square at a time. To make games faster, people modified pawn moves: they could now jump two squares when leaving their home squares.

I hope this helps! Let me know if you have any other questions.