When a camera is selected in the Hierarchy view, its frustum is visible in the Scene view. A frustum is a geometric shape that looks like a pyramid that has had its top cut off, as illustrated here:

The near, or top, plane is parallel to its base. The base is also referred to as the far plane. The frustum's shape represents the viable region of your game. Only objects in that region are rendered. Using the camera object in Scene view, we can change our camera's frustum shape.