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?
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.