By default, Unity only renders objects viewable within the camera's field of view. This is called Frustum Culling and is very common in computer rendering. Occlusion culling is a feature where Unity disables rendering of GameObjects obscured by other objects.
The following image illustrates the difference:
Figure 5.6: Left: Normal scene, no culling
Middle: Frustum culling, only objects within the camera's viewing cone are rendered
Right: Occlusion culling, obscured objects are not rendered
Middle: Frustum culling, only objects within the camera's viewing cone are rendered
Right: Occlusion culling, obscured objects are not rendered
Occlusion culling will not be needed for our simple scene, but larger game environments with many areas are often hampered by performance issues. If this occurs in your environment, try setting up occlusion culling. It is very simple and will greatly improve performance. See the following link for more information: https://docs.unity3d.com/Manual/OcclusionCulling.html.