That isn't true (entirely.) Objects which are outside the viewing frustrum (ie. off to the sides or behind the camera) are culled but everything in the viewing frustrum, whether or not it is occluded, is rendered.
Think of a house for example. You could be looking at a wall and thus cannot see anything else but without any sort of occlusion culling system like a portals engine the game will render everything on the other side of the wall, ending by drawing the wall last on top of everything else.
Think of a house for example. You could be looking at a wall and thus cannot see anything else but without any sort of occlusion culling system like a portals engine the game will render everything on the other side of the wall, ending by drawing the wall last on top of everything else.