the space outside: the area beyond an enclosure, and so the open ground, the open country, or the streets of a town
At its core this word marks one simple idea: the region that lies outside something enclosed. It is the counterpart to whatever is inside a tent, a house, the camp, or the city walls. From that one idea the word fans out in the ways the Bible actually uses it.
In the singular it usually points to the open air just beyond a door or a wall: to step outside, to throw something out, to send a person away from the camp. The prepositions that attach to it sharpen the direction (going out, staying out, being kept beyond a boundary), but the base sense is always the same external space.
In the plural it most often names the streets and public places of a town, the open ground threaded between the buildings where people gather, mourn, fight, and trade, and where bodies lie unburied in pictures of judgment. A related use widens the same plural to the open countryside or fields, the world beyond any settlement at all. These are not different words but the one word seen at different scales: just outside the door, out in the streets, or out across the open land.