This was already thoroughly answered in our Pokemon GO IV Guide article, so just quoting it here:
IV stands for Individual Values. IVs are a mechanism to introduce diversity in a group of same species Pokemon. IV influences various other Pokemon stats, like the amount of Health Points, attack power, etc…
A good metaphor for IV is to think of them like Pokemon genes.
Not all Pokemon are born equal. Some Rattats are bigger, some Pidgeys attack harder and some Kakunas have more defense. In order to facilitate this diversity, IVs were introduced in the original Gameboy games and hidden away from the general player base. There are only 3 main IVs that you should remember:
- Attack IV
- Defense IV
- Stamina IV
IV values go from 0 to 15. IVs are not the same thing as base stats. Even though base stats also depend on the specie, they increase when a Pokemon evolves – IVs always stay the same.
IVs play a big role in determining the true power and strength of every Pokemon you encounter in the wild. They are especially important when comparing same species Pokemon with similar Combat Power.
IVs and possible IV combinations are often displayed with a simple table: