EV training is a way to make your Pokemon the strongest possible. Every Pokemon has an Effort Value that represents a different stat, for example Zubat has an Effort Value of 1 Speed. So when your fight a Zubat, your Pokemon gets 1 Speed Effort Value. 4 Effort Values (EVs) gives your pokemon one plus stat in the kind of stat they collected. So killing 4 Zubats would give your Pokemon 1 higher speed. You can have 510 max EVs of every stat, with 252 in a single stat. So a really fast, really powerful attack Pokemon would have 252 Attack, 252 Speed EVs.