The nature of a pokemon can increase the base stats for 1 stat and decrease the base stat of another. All natures increase & decrease a stat's base by 10%, however some natures have no effect at all. For example, an Adamant nature would increase Attack by 10% and decrease Sp.Atk by 10%. Also, you can tell a nature's effects by looking at the colored text in the stats screen. Red means an increase, blue is a decrease. Note that you have to look closely to tell the difference. Hope this helps. ～Aerostar