EV/Stat F6ing

IVs are randomly generated for all Pokemon, but some of the IVs of the parents are passed down to the offspring. You can breed parents with high initial IVs to increase the chances that the baby will have high IVs. If you're playing in-game, breeding for IVs is an incredibly tedious process, which is why so many people RNG abuse and hack to battle. IMO perfect IVs don't matter all that much unless you need to use a Hidden Power of a specific type. Natures and EVs make a much greater impact on your Pokemon's stats, and as long as you're not using Pokemon with IVs all below 10, you shouldn't notice much of a difference.