iron bars; iron grille; iron lattice
Refers to a physical barrier made of iron bars, such as a window grille, fence, or cage.
The window is fitted with iron bars.
Iron grilles are used on the gate of the old house.
Colloquial metonymy: iron bars as a symbol for prison. Often used in expressions like 鉄格子の向こう or 鉄格子の中.
He is reflecting on his actions behind bars.
If you do something like that, you'll end up in prison.