Back-rank weakness
A castled king behind three unmoved pawns has one escape square: none. Every rook on the first rank is a threat.
White to play
Re8#
The three pawns that look like shelter are the reason the king has nowhere to go.
What to look for
Look at your own king. If f7, g7 and h7 are all still on their squares and no piece guards the eighth rank, you are one rook away from being mated.
When it applies
Every time a rook or queen can reach your back rank, and every time theirs is similarly shut in. Making a single escape square is usually a free move you should spend early rather than late.
How it goes wrong
The escape square has to be safe. Pushing h3 to make luft while the g-file is open can hand them a different mate instead of preventing this one.
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.