To me a GW pro is someone who is helpful, friendly and is knowledgable about how the game works(weapons, mods, skills, quests, missions, monsters and bosses) Not nessacerily knowing everything but enough to show you are expierenced.
Being a pro either means that you pwn everything or that you have a good understanding of the game, are helpful and such. In any case it's a nice compliment to receive.
It means that you can go onto websites and attempt to brag about how good you think you are in order to boost your ego. Basically, it's akin to stroking the old e-peen!
This topic has turned into a pure +1 postcount booster-thread, and it serves no real purpose, nor is it a "basic guild wars question" as the forum description says.