PolygonOccluder3D stores a polygon shape that can be used by the engine's occlusion culling system. When an OccluderInstance3D with a PolygonOccluder3D is selected in the editor, an editor will appear at the top of the 3D viewport so you can add/remove points. All points must be placed on the same 2D plane, which means it is not possible to create arbitrary 3D shapes with a single PolygonOccluder3D. To use arbitrary 3D shapes as occluders, use ArrayOccluder3D or OccluderInstance3D's baking feature instead.

See OccluderInstance3D's documentation for instructions on setting up occlusion culling.