a bar: the strong wooden or metal beam used to lock something shut or to bind parts firmly together.
This is a plain, concrete word: a bar. It names the strong wooden or metal beam that holds something firmly in place, whether by locking it shut or by binding its parts into one. It is a piece of hardware, not an idea, so most of its appearances simply describe how something was built or secured.
The same object turns up in two everyday settings. In the tabernacle it is the long pole run through rings to lock the standing frames into a single solid wall. At a city gate it is the heavy beam dropped across the doors to bar the way in, which is why a town with strong bars counted as safe and a town with broken bars had fallen.
From that gate image the word reaches into poetry. God sets a bar across the sea to keep it in its bounds, and Jonah, sunk to the roots of the mountains, feels the earth close its bars over him like a locked prison. The picture never changes: something shut fast and held.