Rooks are the hardest pieces to develop: they start in the corners, and pawns block them everywhere. An open file, one with no pawns of either color on it, is a rook's highway: control it and your rook sees all the way into the enemy position, ready to invade.
This is one of the simplest positional rules with the highest payoff: when a file opens up, race your rook to it first. A rook that controls the only open file often decides the whole endgame by itself, because the opponent's rook is stuck reacting instead of invading.
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Both d-file pawns are gone, it's the only fully open file on the board. Neither rook has claimed it yet.
Seize the open file before Black does.
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