Unlike the GoH which is usually, but not always a pro, the FGoH is always a fan -- which of course does not preclude that person also being a pro, as long as the honor is for their contributions to fandom specifically. Many well-known pros are also fans and have been honored as Fan Guests of Honor.
See GoH for an extended discussion which applies to all kinds of GoH.
If a Fan GoHship is an honor, the Worldcon Fan GoHship is the ultimate accolade and as such has a much higher standard than FGoH at a regional convention. Like any Worldcon GoH (see the discussion under GoH) a Worldcon FGoH is fandom's equivalent of a Nobel Prize.
A Worldcon FGoH should have been active in fandom for at least thirty years and should have made major (positive) contributions in more than one area of fandom. It should never be given for sheer longevity, and not normally for someone who excels in only one thing.
See more about Worldcon GoHs at GoH.