nothing really, it just tells you how many achievements you did while playing a game, and 10g counts for 10 gamerscore points.
Unlike people with 10,000 to 20,000 gamescore, those guys are just nerds, they don't have a life, they are virgins, they will never get laid.
Me, I just have about 2525 gamescore, from playing the xbox 360 since it came out like 3 years ago.