Individual Values (IVs) are a set of hidden values for each of a Pokémon’s six key stats (HP through Speed). IVs range from 0 to 31. The higher the IV, the better the stat. Like Natures, IVs are determined when a Pokémon is caught/obtained and cannot be changed (kind of). Unlike Natures, you cannot directly see a Pokémon’s IVs.
You can consider IVs a measure of a Pokémon’s natural talent. It’s similar to real life, where some people may be smarter than others or more athletic. In terms of Pokémon, there may be a Pikachu that has more Speed than another Pikachu, but their Defense stat might not be as good. Also, just like real life, you can somewhat cover for poor IVs with sufficient training.
The IV Advantage
Before you get too worried, knowledge of IVs isn’t required for the main game–it’s mainly for people who like to min-max. For instance, when battling with other players. IVs can have a similar contribution to Natures, especially at higher Levels. So they’re well worth paying attention to.
At Level 100, 1 IV is equal to 1 additional stat point. Looking at the two extremes, a Pokémon with 31 IVs in one stat has a 31 point lead on a Pokémon with 0 IVs in the same stat. This may not seem a lot, but it could make all the difference in high level play. At lower Levels, the contribution from IVs is proportionally smaller.