Question

I have seen a lot of questions and answers with the word uber, but I don't know what it is.

Answer 1

Uber is a tier where all Pokemon can be used.
This is the tier for Pokemon like Giratina, Ho-Oh and Rayquaza and the likes that are just too powerful for all the other tiers.

Smogon's Uber tier
The PokemonDB's Uber tier

Answer 2

Uber is a competative Pokemon tier
It is the highest tier and normally consists of legends
This tier is for Pokemon that are overpowered
Here is the Pokemon data bases tiers

Comments

So it's basically just really strong Pokemon?

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And wobbufet

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wobbufet is considered really strong.

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Because its power to hold out with so much hp then counter or mirror coat