From Halopedia, the Halo wiki
The Hero medal is awarded in Halo 3: ODST and Halo: Reach Firefight mode for being the only player left alive, and surviving a wave. Upon the next wave, your team is revived, and you are awarded with the Hero medal.
Originally, According to the Bungie Day (7/7/09) Podcast, the last man standing would be able to revive their entire team by earning three hero medals. However, this feature was removed in the final release of the game.