Open file
Rooks need files with no pawns. Occupy the file, then contest it — the side that doubles first usually keeps it.
White to play
Rd1
d is the only file with no pawns on it. Whoever puts a rook there first decides who gets to use it.
What to look for
Scan the eight files for one with no pawns of either colour on it, and for half-open files with only the enemy's pawn.
When it applies
As soon as pawns come off. A rook on a closed file has no future move; the same rook on an open file threatens everything on that file for the rest of the game.
How it goes wrong
Taking the file is not owning it. If they can meet you on it and trade rooks, the file becomes nobody's — which suits whoever had the worse position.
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.