Hey guys...what exactly does FSUIPC does?

I guess Pete Dowson, the author of FSUIPC, will drop by soon and he can definitely answer your question to much more detail (and he will correct me, if I say some stupid things here), but in a nut shell that program works as an interface between addons and FSX (freeware version)

Since it's an interface program, you can do a lot more with it, e.g. assign joystick axes, buttons etc, and you can even write some extra functions. So basically FSUIPC connects any sort of third party "addon" (be it hardware or software) to FSX by using its own piece of code and it helps overcome some limitations of FSX. And then there's also some things like smoother transitions between two weather stations, the OOM warning sound, which can be very useful.

For a very detailed insight in what exactly FSUIPC does or is capable of doing, you could also download and install the freeware version and read through the extensive manual.