From Halopedia, the Halo wiki
MVP is an achievement in Halo 3 and Halo 2: Anniversary. It is unlocked by being named the MVP in a matchmade team match. To be MVP, the player must be the top player on the winning team. In Halo 3 the match must be in a ranked team-based playlist.
In the original game, it is worth 5 Gamerscore and is represented by a blue-and-white circle with a Spartan running with a weapon. To his right is a red star and the letters "M-V-P". In Halo 2: Anniversary it is worth 10 Gamerscore and is represented by a screenshot from the game's epilogue.
"MVP" stands for Most Valuable Player.