The disappearing pawn—why did it get captured?

Why did my pawn disappear when the enemy pawn passed it????

This capture is called an en passant capture, and it can occur when one of your pawns moves two squares on its first turn, and the square it passes over is attacked by an enemy pawn. This enemy pawn is entitled to capture the moved pawn "in passing" as if the latter had advanced only one square.

I hope this made sense… before I knew what the en passant was, I was like, “WHERE DID IT GO?????!!!!!!” And made a whole iMovie about it saying the odds were in the other player’s favor.