This achievement from the Legends and Killers DLC is unlockable once you reach rank 50 in multiplayer, which happens once you accumulate 337,110 XP. To unlock it, go to the outfitter menu and press Y to pass into legend. You will be prompted twice before the action is carried out.
When you pass into legend, your XP resets at zero. All of the challenges - e.g. Lawbringer, Bounty Hunter, etc. - are reset. You lose the weapons and mounts you have unlocked, but you do NOT lose the characters and titles you have unlocked. You also do not lose any game types/maps you have unlocked. For example, if you have unlocked Advanced Co-op missions, they will still be unlocked after you pass into legend. If you have earned a golden gun, it will still be golden in any game types/maps that have it, and when you unlock it again in free roam, it will be automatically be golden.
If your goal is to reach legend level 5 in multiplayer as quickly as possible, you will generally want to pass into legend as soon as you hit rank 50, because any XP you earn at rank 50 are wasted. The main reason you might not want to pass into legend immediately is if you are close to getting a golden gun. If you reach rank 50 and you have, say, 82 headshots on a specific gun, you might want to hang out at level 50 for a while and get those last 18 headshots before passing into legend, because once you pass into legend, you will start back at zero.