each nature affect the stats of pokemon...let say
lvl 20 pikachu adamant nature:attck 20,def 15,special attck 15,special def 17,speed 25
lvl20 pikachu lvl 20 brave nature:attck 20, def 15,special attack 17,special def 17,speed 23
adamant nature decrease special attack and brave nature decrease speed...
hope u understand what i mean...