A vidmaster must play on the hardest difficulty level, never shoot when he (or she) could use grenades, never sprint The "vid" refers to the recording the player makes of their actions in the game that proves their skills. That was in marathon in halo it was a bit more different but ultimately rewards you with armor that was quite unique "recon"

Ok thanks for the explanation

Someone who is really good at bungie games. I can't recall if it started with Marathon or earlier. I was never one of them :)