Reciprocity is a word that frightens some people only because it sounds complicated but it needn't be. It's as simple as this: you punch me on the nose, I punch you on the nose back! I reciprocated your action
Seriously though, understanding this principle is important for new SLR users seeking to get to grips with using shutter speeds, apertures and ISO so i have made a video tutorial on the subject which I hope will be of help to some.
It's all a bit fruity at the beginning but stick with it, it (hopefully) all becomes clear
Cheers,
Rob