Backlighting can be a very powerful tool in your arsenal of photography tricks. If you've never done it before it basically consists of lighting the subject from the back, which is I guess where they got the name from. It is typically used on smaller subjects that you would shoot with a macro lens, and depending on the subject and its position you will get a wide range of results. Fundamentally it's quite simple, just put the light behind the subject with a small aperture and quick shutter speed and fire. But in practice, at least in my experience it tends to be quite a bit more difficult.

Before you grab your camera and go looking for things to light up it helps to have a couple tools to take down with you. One is a snoot and the other would be a small tripod for your strobe/snoot but you can get by without the second thing. My snoot for my Inon Z-240 is just a piece of plumbing PVC with a slight taper the end and it works wonders. 

As I said you can back light just about anything but for me the best are subjects that are semi-transparent, like leaf scorpion fish or some ghost pipe fish, so the light penetrates though them and gives a very unique glowing look. If you back light a subject like a frog fish with a more solid body you will only what looks like a silhouette effect where only the edges are highlighted. The best subjects for backlighting will be the ones that are sat on top of something with clear space all around, this allows allows you to easily isolate your subject. If you subject is hiding amongst the corals it makes it really difficult to isolate the subject from the background even with a snoot.

Once your've found the perfect candidate for backlighting carefully place your snooted light on full power (Inon Z-240) so that the light is behind and to the subjects front so that it's pointed at it's head. You'll need to detach your strobe from your housing of course and I don't put it directly behind unless its below the subject other wise I find it gets blown out and to keep the strobe itself out of frame. Depending on what you want to accomplish you may need to move the light around a bit, I like it when the light penetrates through the eyes and makes them glow red. The most common background associated with backlighting is the all famous black background which can be achieved with a quick shutter speed and small aperture. I find that 1/250 and f-13 and smaller do the trick on a sunny day.

Oh, and a stationary subject is of course the easiest subject for back lighting!