With Raichu lv X, you get to attack an additional time.
So, let's say my opponent has Kricketune (80HP) out.
I level up my active Raichu to Raichu lv X. I then use Raichu lv X's attack, "Voltage Shoot", to KO his active Kricketune.
Now, I should be able to attack an additional time...