Smogon classifies pokemon in tiers depending on how good they are in the metagame.
ubers is the tier with the best pokemon (legendaries, good megas).
OU is the main tier with pokemon that are very good but not as overpowered as legendaries.
UU, RU, NU, PU are tiers with pokemon that are not that viable in the competitive environment.
These tiers *roughly* translate to in-game viability, but certainly not to 100%.