Original: Uberfic is a variety of fan fiction in which the characters live in an alternative universe. The characters and events are true to the original canon but usually in a different time period, often as the ancestors, descendants or reincarnations of canon characters.
Simplified: Uberfic is a kind of alternative universe fan fiction in which characters or events are portrayed somewhat closely to original canon but usually in a different time period, many times featuring the ancestors, descendants or reincarnations of canon characters.