Originally, pawns could only move one square. However, to speed up the openings, they started playing so that the first move pawns made could be 2 squares.

That created a dilemma, though; now, if an opponent pawn was in position to capture yours if it moved forward one square from its start, you could evade capture by moving two squares instead. So they created the en passant rule, which allowed the capture of a pawn moving two squares as if it had moved just one square instead.