FILL FLASH!!!!!!! What is it? Fill flash is using added light to lighten the shadows on your subject, or to illuminate your subject because the dynamic rage in your image is to great for your camera to capture.
Personally i think it dose deserve to be in big capital letters with exclamation marks it is that important. I think its the next thing to learn after exposure and computation, its so easy to do weather your using a reflector, pop up flash or a top of the line speed light, it is a simple uncomplicated way to make your images 100% in just a few hours practice.
The simplest way is to just set your flash to auto and fire away, thats it. You can play with the plus and minus to get the most pleasing look but 9 out of 10 times on a nikon ttlbl setting works fine.
After you have mastered that you can start playing with it off camera flash, this is the enjoyable bit and the basis of all strobist work. it is relatively simple, if you start with your ambient exposure then dial in your flash to give the best look.
Next its adding diffuses to give a better quality of light and your really in to the strobist lighting. Stick with the same principals start with your ambient and you will do just fine, remember to keep in mind your flash will need more power as the diffusers eat up some of the light.
Now your ready to start adding more lights what then is no longer fill flash. So go crazy and see what you can come up with.
Thanks for reading Ade_1