>>27635505They're hidden stats basically. Every pokemon has 30 possible Attack/Defense points and 15 possible Stamina points.
You catch two Ponyta of a similar CP. When they're caught their IVs are randomly generated. One of them has 7/30 and 10/15. The other has 20/30 and 10/15. The second one is better because its hidden stats are higher. These stats subtly affect a pokemon's performance against other pokemon. You want to aim to have the best IVs possible if you want your pokemon to be used to its maximum potential. It's possible to get something with 30/30 and 15/15 IVs, like pic related.
Hatched pokemon always have good IVs. Randomly encountered pokemon's IVs are random.
