We play with all the rules of chess at our events: touch-move, touch-take, castling king-first and en passant. 

The last is the hardest to learn!

It's easier to remember if you remember why it was invented.

Pawns used to move one square at a time.

This meant that a white Pawn could not sneak past a black Pawn, without Black having a chance to take it.

But then the rule was introduced with a double-square move as a Pawn's first move.

Now if a brave black Pawn gets close to the white ranks, waiting to take a pawn, a white Pawn might be able to sneak past it by moving two squares.

So, it is allowed for the black pawn to take a pawn that tries to sneak past it, on its next go (and only on its next go).  You pretend it moved only one square, and take it on that square.

That's taking in passing -- en passant. 

• It's only for a pawn taking a pawn
• only taking a pawn that's trying to sneak past by moving two squares on its first move
• and only on the next move.