The respec was for vetran players when the CU first came out. Since they completly changed the way the professions worked they gave 2 weeks of respec where you could change your template around to try different professions without having to grind them out.
Anyway, when the 2 week peroid ended may people were caught unaware and stuck with a profession they didn't want. (Even though they knew it was to be 2 weeks, apparently they can't count to 14)
So the DEVs gave everyone one last respec to be used whenever you wanted. That's what the respec token is. If I were you I would save it until you master a profession or 2 and then use it if you want to change to a different one. The only problem with this is that you don't get the master's badge when you master a profession if you still have any respec left.
As far as respecing, you trade skill boxes in one profession for skill boxes in another one. You can't get more boxes than what you had and you can't get elite profession boxes for basic profession boxes. Also you need the pre requisite to get a box.
So if you had some marksman or brawler boxes, you couldn't turn them in for pistoleer, even if you had the pre requisite.