A slot in a ground plane radiates like a magnetic dipole, on both sides of the ground plane. If you don't want it to radiate on both sides, or you have something filling the gap on one side, then you have do do something other than just fill in the slot with metal, which will stop it working. A cavity behind the slot is a good solution. A simple resonant cavity will work for a narrow band antenna, looking like an open circuit across the slot at its resonant frequencies, and a loaded cavity will give wide band performance, but absorb half the power.