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