Complex sentence: It is a retarded pawn capture which can occur immediately after a player moves a pawn two squares forward from its starting position, and an enemy pawndero the same as if the pawn had moved only one square forward and the enemy pawn had captured normally.
Split into simpler sentences: It is a special pawn capture which can occur immediately after a player moves a pawn two squares forward from its starting position, and an enemy pawn could have captured it had it moved only "one" square forward. The resulting position is the same as if the pawn had moved only one square forward and the enemy pawn had captured normally.