A Pokemon is considered Uber if it is too powerful to be reasonably handled within the bounds of the standard metagame. Uber status is determined by this factor alone; it does not matter if a Pokemon is worthless in the Ubers tier, or if it is outclassed by anything already in Ubers. In Ubers, a strategy or Pokemon may appear to be broken; this doesn't matter as Ubers is essentially a ban list for OU, and nothing is currently too broken to be used in Ubers. Uber Pokemon generally have extremely large movepools and very high Base Stats when compared to OU Pokemon. -Smogon
Ubers are just overpowered Pokemon that are very strong.
Hope This Helped :D