http://1.bp.blogspot.com/_SMOTQp0_pAE/SmcHnv1RXfI/AAAAAAAAAM...
A 'flash head' refers to a kind of electronic flash unit, in the form of a 'head' that can be fitted onto a stand, that is very commonly used in studio work by profesional photographers (do a Google image search to see plenty of examples).
The disadvantage, however, with this kind of flash, is that you can't get an impression of what the lighting is going to look like in the final image, as the flash is too short and bright for the eye to properly assimilate it; so these units also include an ordinary incandescent lamp, know as the 'modelling lamp' (or light) which can be left on continuously so that the photographer will be better able to judge the final effefct of her/his lighting; you can see this lamp very clearly in the picture to which I've posted the link above.
I'll leave it to my FR colleagues to give you the proper technical term.