The Key is an achievement for Halo 3 and Halo: The Master Chief Collection. In the original game, it is worth 40 gamerpoints and is unlocked by beating Cortana on Normal, Heroic, or Legendary. In The Master Chief Collection it is only worth 10 gamerpoints, though it can also be unlocked on Easy.
In the original game it is represented by a blue plate with a roman numeral VIII on it. In The Master Chief Collection it is represented by a screenshot of the level.
Note: You must finish the level completely through, or the achievement will not unlock.