This examples shows the use the Cells&Portal culling optimization. In order to understand the meaning you should change to Wireframe rendering using the selections below and move around in the 3d scene. You should now notice, that geometry which is further away appears/disappears depending on your current view. The reason is, that this scene is divided in different Cells, where the geometry of another cell is only rendered, if it is visible through a doorway (which is defined as a so called "Portal")

Change rendering: