IVs are randomly generated "boni" which are added on top of your Pokémon's base stats.
A golbat has 164 attack, 164 defense and 150 stamina as base stats. So a 0 attack IV golbat has 164 attack whilst a 15 attack IV golbat has 179 attack. Thats a 10% gain in damage. (Of course your moveset will affect the damage aswell) But not only your damage is different, your CP cap will be affected aswell.
Overall, IVs arent that important in earlier levels. Just make sure you evolve and powerup Pokémon with at least 60/70% IV and hope you get a good moveset, because good movesets are more important.
