So, I'm sure you know what stats are. Two of them include evasion and accuracy.
Evasion is, according to Bulbapedia:
The evasion rate (Japanese: 回避率 evasion rate), or evasiveness, of a Pokémon's move determines its probability of missing the target.
And accuracy is:
The accuracy rate (Japanese: 命中率 accuracy rate) of a Pokémon's move determines its probability of hitting the opponent.
Therefore, if I use Double Team, for example, that increases my evasion, meaning that my chances of avoiding the attack are bigger. If I use Flash on the opponent, their accuracy decreases, meaning that their chances of hitting me are smaller.
Hope I helped :]