In every battle, you gain EV points. For example, defeating a Bidoof gives your Pokémon 1HP EV. Same is true for Starly and Speed. Beating Spiritomb gives you 1 point from both Sp.Def and Def.
EV training is when you're training your Pokémon on only Bidoof for HP or Machop for Attack. So, it is better to keep training on lower level Pokémon because you get less EXP and same EVs, so when level-up, your Pokémon gains MORE new stats.
not only, but mostly