[Glow=#9932CC]Krysi is correct, it is all explained on the page. But I'll try to make it simpler.
When you have a Playerdex Bank Account, at the end of every month, a small percentage of what you spent that month in the Playerdex Mart is given back to you. Upgrading to a better bank account increases the amount you get back. You also get a few extra Tokens when you upgrade [/Glow]