Although there have been a number of adaptations of the character, Man-Bat is traditionally the scientist Dr. Kirk Langstrom who transforms himself into a man-sized bat after he tested an experimental formula on himself.
When in his bat form, he’s lacking in all intelligence and self-control, and he’s prone to deadly rampages.
Man-Bat could be what motivates Bruce to make the jump from masked vigilante to Batman.
A big question now is how closely Gotham will follow Man-Bat’s comic origins.