A garrison was a contingent of troops stationed at a specific location for defensive purposes.
In early Klingon history, 500 warriors stormed the Great Hall at Qam-Chee. The city garrison fled before them with only Kahless and Lukara making a stand. (DS9: "Looking for par'Mach in All the Wrong Places")
Later that year, the weapons depot on Adarak Prime was protected by a single garrison of Cardassians. Damar and his rebels were reluctant to attack the depot and kill their own people but were convinced of the necessity to do so by Kira Nerys. (DS9: "When It Rains...")