Pokemon IV stands for Individual Values. As explained by Professor Oak on this article, it can also mean the Pokemon's genes. No Pokemon are created equal. They have their own stats aside from CP, HP, etc. A level 5 Ratata doesn't mean it has the same stats like all level 5 Ratatas, sometimes a good IV Ratata can have a slight advantage versus another Ratata of the same level. You can either have the worst Pokemon or the best possible Pokemon there is.
Read on to find out more about Pokemon Go IV in this wonderful comic we found:
Pokemon Go IV according to Professor Oak!
In short, there is something that you can get the moment you "catch" you Pokemon in Pokemon Go. And that, together with the Pokemon's CP and Level will determine whether it is a Pokemon you will use to battle against gyms, or just a Pokemon that you will transfer to the Professor.
There used to be a site, Pokeadvisor that deals into this stuff. However, it looks like Niantic banned the site, but we did find a good working site:
You can also try apps on Google Play and App Store but be cautious in downloading them though.
Who would've thought Pokemons can be this complicated? Well, I guess it can be complicated if you chose it to be. And I know most of us usually just look at the CP of a Pokemon before we transfer the weakest to Professor Willow. Looks like it is going to be a game changer to some players who really wantsto be the very best! Or I guess they already know that.
Good luck on finding a good IV Pokemon in Pokemon Go!