From their about page:
Flattr is the worlds first social micro-payment system.
In short, you create an account, add to your account at least 2 euros each month (about 2.80 usd at the moment), and select one or more things (official term). Your 2 euros (or more, if you like) will split among the things you have flattered (again, official term).
At the moment we have 2 flattrs (not sure if this is official term). We have to pay 2 euros each month to keep our account alive, so we hope we will get at least that much.
I promise we will not spend all of the money on beer. :)