Anti-gravity platforms are a piece of technology built by the Goa'uld.
Their main function is to keep Ha'tak vessels under construction above the ground until they are fully built and operational. They usually encompass the Ha'tak vessel itself. There are around six platforms that help the ships levitate. However, they are in a state of balance, and if one platform is disabled or shut down, the rest of the system would be unable to compensate, and the vessel will plummet to the surface. (SG1: "Orpheus")
One such platform was encountered on Erebus, which was used to levitate a near complete Ha'tak vessel near a Jaffa labour camp to mine the naquadah to build the ships. Major Carter and Doctor Jackson went aboard the ship and planted C-4 on one of the panels, which would disable a section of the platform, and the ship plummeted to the surface. That was used as a distraction to liberate the camp and defeat the enemy Jaffa. (SG1: "Orpheus")