Return is a move that becomes more powerful when the Ponymon is happier with the trainer. This means the the level of happiness determines its attack power.
To calculate its attack power you just simply have to multiply the happiness by two and divide it by five (for example, if the happiness is 200 the attack power will be 200*2/5= 80)
The happiness can range between 0 to 255 (although reaching 0 it's almost impossible in Gen III games) so the attack power will be anything from 0 to 102
Ponies that can learn this move are:
"This attack move grows more powerful the more the user likes its TRAINER."