In the halo reach achievements what does it mean when it says Noble 1 or Defiant?

Contrary to what Jersey Rebel says, You DO have to get the related map packs to be able to get the achievements even though most aren't map specific. Otherwise you can fulfill the criteria for the related achievement 50 times over, particularly the one for getting a kill in Rocket Race in matchmaking(Rocket Hog Race in Multi Team), and you still won't get the achievement. By downloading the map packs, you are also downloading the the ability for your system to recognize you getting the associated achievements. It's the same deal for map packs and new achievements for DLC across every single Xbox 360 game.