IVs are Individual Values for Attack, Defense & HP that each individual Pokemon has. Every Pokemon species has base stats that are the same but up to 15 extra "stat points" can be added for Attack, Defense and HP values for each individual Pokemon. When you click "Appraise" on a Pokemon, your Team Leader will give you a general appraisal based on these extra "stat points." The difference between a Pokemon with 0-0-0 stats and 15-15-15 is only 5% - 10%. In general IVs aren't terribly important, but if you want the strongest Pokemon possible, IVs matter.