https://en.wikipedia.org/wiki/MOSFET
basically the type of semi-conductors used in the RR. They have lower volt drop across them and produce less heat etc, better reliability and more stable voltage regulation typically. A lot of bikes use them now, they "usually" have 2 sockets on them, one with 3 wires for the alternator supply, one with normally 2 wires for + and - output (may possibly have a third but usually just 2).
I got mine used off ebay, there's no hard and fast guide for which bike model to go for. The MOSFET ones usually have a code on them which will be FH010 or FH012 or FH020 etc. The higher numbers usually mean greater current capacity I think, or later versions.
You'll usually need to alter the wiring a bit on your bike to suit, normal 6.3mm spade connectors will go straight on the contacts as long as you know which is which, or you can get the correct sumitomo (?) sockets.
Try a search for Shindengen on the net for info, Eastern Beaver can be a useful source of info and connectors if you want to go that route (I haven't).