This would be a Side Dump Trailer. Dumping is a very efficient way to unload bulk materials at the destination. Plop and it is gone. Classic Dump trucks dump out the back and that is fine. There is a problem however when you try to make the truck into some sort of a combination (add a trailer). They make dumping semi-trailers, but they are very unstable while dumping. It is easy enough to tip over a traditional dump truck on uneven ground while dumping, but attempting to dump a semi truck end dump under any the the most favorable conditions (a concrete pad) is not acceptable. Belly Dumps are popular for trailers but they imply that your load is something that you don't mind driving over after you drop it (gravel maybe). But suppose your load is land clearing debris (stumps, etc) or building debris, or big ugly rocks etc.?
The side dumping trailer (shown here in the dumped position) allows you to deposit your load of what ever beside the truck. You don't have the stability problems while dumping associated with the long end dump, and are spared the need to run over your load as is required with a belly dump. This is certainly not the first choice for graveling your driveway, but a good choice as a debris trailer.