A backhop is about a basic move done on a bike in the sport of bicycle trials. The idea of a backhop is to balance on the back wheel, with the front wheel above the ground, and then pull the back wheel up using your feet. It's similar to bunnyhopping or a sidehop except that it's done on the back wheel. The end result of a backhop is that you go backwards, at first, and as you get better you can use it to balance in place on the back wheel. To do a backhop properly I would recommend learning to nosepick and to bunnyhop first.
So, to make these skills come together for a backhop try these steps. First, start by riding at about a walking pace, standing up. From here do a nosepick, that is lock up your front wheel to raise your back wheel off the ground. Then, as your back wheel comes down shift your weight back. When you're back wheel reaches the ground you're front wheel will start to rise. Now tilt your feet forwards (toes lower than heel), and pull backwards and up. This should raise your back wheel off the ground. Now just work on this until you can do more than one in a row.
Because you move backwards while backhopping it isn't very useful for actually going places. For example, if you want to drop off of a picnic table you're going to want to be able to see where you're going. So once you get good enough at backhopping that you can do about 10-12 in a row learn to backhop forwards. This will allow you to do many more things than before.
For help with any of this message me and I'll be glad to answer any questions. To see videos of this move and others (in quicktime) see www.trials-online.com