The 7th rank (2nd, from Black's side) is where a rook does maximum damage: it attacks the opponent's pawns from behind, where they can't be defended by other pawns, and it cages the enemy king on its back rank at the same time. Grandmasters have joked for a century that a rook on the seventh is worth a pawn all by itself.
The typical plan: win the open file first (previous lesson), then use it to lift the rook into the seventh. Two rooks on the seventh, "pigs on the seventh," are usually decisive, gobbling everything in sight while the enemy king watches from its cage.
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White's rook owns the open d-file. The 7th rank, and Black's undefended queenside pawns, are one move away.
Invade the seventh rank.
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