The BSP tree, or Binary Space Partition, splits your scene up in to sections depending on how you use the settings. Normally when rendering the camera shoots out rays and then waits to find out if they hit anything. When using the BSP settings the renderer looks first to each of the partitions and asks if any rays in that partition hit anything, if not it moves on to the next partition saving the time it would have taken to ask each individual ray. That is the BSP tree in a nutshell. In my own work i havent found that changing any of the BSP settings have given me any hudge advantage. Maybe a couple of seconds a frame if that. But I havent played around with it very much. Hopefully when the new video tutorials come out they will cover a little bit of the BSP stuff.