Equivilent to WOW! Usually used when excited or amazed . Sometimes just to irritate the shit out of someone when one is trying tell you something lame that he/she finds interesting .
It's easiest to describe Fuiyoh as an equivalent to poggers. It's essentially an exclamation of general positive emotion. To name a few, it could be excitement or being impressed or, of course, sexual delight, because this is Urban Dictionary.