Rook activity
A rook that sees nothing is worth less than a knight that sees six squares. Activity outranks material in rook endings.
White to play
Rd7+
From d7 the rook checks the king and attacks two pawns. On d1 it attacked nothing at all.
What to look for
Count the squares each rook can legally move to. A rook defending a pawn from behind it usually has one; the same rook attacking from the seventh has ten.
When it applies
In every endgame with rooks. Given the choice between defending a pawn passively and giving it up for an active rook, the active rook is right far more often than beginners expect.
How it goes wrong
Activity is not the same as adventure. A rook that runs into enemy territory and gets trapped behind a pawn chain has spent its activity on nothing.
Where it came up
Not cited in your current game.
Practise it
No positions yet. When you miss this idea in a game, the position it cost you shows up here — and in the practise set — with the answer hidden.