Original: That enemy pawn, which would have been able to capture the moving pawn had it advanced only one square, is entitled to capture the moving pawn "in passing" as if it had advanced only one square.
Simplified: The black pawn, which would have been able to capture the white pawn if the white one had moved forward only one square, can capture the white pawn in its next move - "in passing" as if the white pawn had moved forward only one square.