With a blocked pawn pair, the side that owns an extra pawn behind the front uses it as a reserve tempo to win the opposition at the decisive moment.
White has pawns on e4 and d2 against Black's e5; the kings are on d1 and d6. The e-pawns block each other, so the fight is about which king reaches the enemy pawn first, and that fight is decided by the opposition. White's d2-pawn is the extra tempo that settles it.