Like Call of Duty's Prestige or Red Dead Redemption's Legend, Rebirth allows you to start over at Level 1. This can only be done if you own the Rapture Metro DLC. It is the only way you can get to Level 50 AND Rebirth (the Sinclair Solutions pack allows you to get to level 50, but not Rebirth). You get a special mask to show off your "bravery" but you lose almost everything; Plasmids, Tonics, Weapons, ADAM, and even your spot on the leaderboards! A patch is suppose address the leaderboards, but for now, be cautious when doing Rebirth unless you strictly want the achievement or don't care about your leaderboard rank.