Occlusion Culling

The general idea of visibility and occlusion culling methods is to reduce the number of visible objects at any given time with the goal of gaining performance.

Visualize Occluded Primitives - When enabled, a green bounding box is drawn around any occluded Actor.

Freeze Rendering - You can "freeze" the rendering state for Actors in your Level while working in the Editor so that you can freely move around within the Level viewport and inspect occlusion results.

More Info About Occlusion Culling:

https://docs.unrealengine.com/5.1/en-US/visibility-and-occlusion-culling-in-unreal-engine/