The Uber tier is effectively a banlist for OU, however this does not mean it is not, or cannot be used. Pokémon in the Uber tier are deemed too powerful or centralizing to be used fairly among other Pokémon.
While most Pokémon in the Uber tier are there due to high stats, Wobbuffet is often classed as an Uber because of its Shadow Tag ability, along with Encore, Counter and Mirror Coat; this lets it trap any opponent Pokémon and lock them into any move, so that the player knows what their opponent will do next, which allows the player to either switch out Wobbuffet knowing what the opponent will do, or use Counter or Mirror Coat to damage the opponent. Wobbuffet's pre-evolution, Wynaut, is sometimes banned for similar reasons. Other non-legendary Pokémon such as Garchomp and Salamence are sometimes considered Uber due to their movepools, high base stats, and type.
Abilities can also be Uber. If all Pokémon with a certain ability are considered too powerful with the ability but, if they have other abilities, are not considered too powerful, then it is the ability that is banned, not the Pokémon. The only banned ability so far, in the case of Smogon's rules, is Moody, banned because it allows a Pokémon to sharply raise its stats, including Evasion, every turn while using moves like Protect and Substitute, and outweighing the drawbacks of the ability.