The official meaning of "farkle" is a functional sparkle. The Harley and goldwing crowd bolt on a bunch of cute looking chrome stuff that makes thier bikes different looking and unique. but for the most part the additions are just useless chrome sparkley junk.
Therfore a Farkle is a add on to your bike that is actually functional in the operational or comfort areas.
So if you want to add some neon lights and a live to ride 4lb platter to the side of your engine that is a sparkle.
A fender extender, radiator guard, and bar risers are farkles.