[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.