I have one of these. The goal is to capture all of your teams monsters, and raise them to their highest level. Basically you scan barcodes, and get one of 4 things:
1. Battle an enemy monster.
2. Capture one of your monsters.
3. Get a poison item (not good)
4. Get a helpful item: medicine, magic spell, weapons.
I cheated and got all the barcodes for each monster, but if you dont cheat it's fun scanning and seeing what you get.