sigh
A tier is a group of Pokemon. When you battle in OU, you can only use Pokemon in OU and below. That's to prevent something like Magikarp vs Arceus, or Palkia, or Diagla!
I won't provide a link to Smogon, but you can find the tiers on Smogon. Just Google. Smogon divided the tiers for you, it's all done!