a garrison: armed men planted in a place to hold it for the ruler who put them there
This is what a king leaves behind after a victory. Once David had beaten Aram and Edom, he "put garrisons" in them, and the point is not the buildings but the control: from then on those peoples brought tribute and served him. The word names an armed presence set down in a place so that the place stays yours.
The Hebrew is vaguer than the English word "garrison" suggests. It belongs to a family of words about being set or planted in position (נָצַב), and it never says whether the thing planted is the soldiers, the fortified post they hold, or the officer who commands them. A close relative (מַצָּב) names the station itself, and the two are near enough that the same Philistine post at Bethlehem is called by one word in 1 Chronicles 11:16 and by the other in 2 Samuel 23:14.
Four of the ten places this word appears describe the Philistine post inside Israelite country in the days of Saul and the young David, and there it carries real weight. Its presence in Israelite towns is the standing evidence that the Philistines, not Israel, hold the land. When Jonathan strikes it, the whole country hears, and the war starts.