An occluder is an object that is used to cull objects' surfaces, bounds of which are not visible behind it.
Occluders can be highly effective in case of complex environments where there are many objects that occlude each other and are costly to render (they have a lot of polygons and/or heavy shaders). However, effective culling is possible only if objects are not too large. If objects are big and have a few surfaces, it is likely that an additional performance load of an occluder will not pay off.
Occluders are rendered by the CPU and stored in a separate buffer. Only the occluders that are displayed in the viewport are saved to the buffer. To display the buffer, you should pass 1 to the render_show_occluder console command: