Basically lube for your strings. Supposedly keeps them clean as well? I've used this for years, and I'm not sure I can tell any difference, but then again, I haven't been using it for cleaning.
What it WILL do exceptionally well is temporarily counteract the friction you'll get from a set of strings after playing them for a while. If you're showing up for rehearsal and you've forgotten to put on new strings for the 9th consecutive week, Fast Fret will at least make your formerly fresh set of strings (now borderline barbed wire) playable for a few hours.
And a can lasts forever. As long as you don't lose it you probably won't have to replace it for several years.