I put 20euro into my Flattr Account, and choose to spend 2uero a month. As I browse the web, when I see content I like, and click the 'Flattr' button, which is like the Facebook like button. At the end of the month, everyone who's button I clicked gets an equal share of 2euro.

I can spend whatever I want monthly, this is just what I actually do.

I think the home page sums it up fairly well, the only thing missing would be to maybe use the words web content or something similar to convey that users are choosing to donate to producers of content that they like. That's how I understand it although I think I watched the video a while back so have a bit more context.