it's the type of attack. attack stats help your physical attacks like stomp, megapunch an shit like that. special attack is te opposite. it helps your special attacks like fireblast, thunder, icebeam and shit like that. the defense stats are the same for each aswell. defense let's u take less damage from physical attacks and special defense let's u take less damage from special attack.
trivia!!!
in the first generation there was no special attack or special defense, it was just attack, defense, speed, and special