Flattr is a micropayment system - more specifically, a microdonation system.
Flattr is a project started by Peter Sunde and Linus Olsson. Users are able to pay a small amount every month (minimum 2 euros) and then click Flattr buttons on sites to share the money they paid among those sites, comparable to an Like button with money on top.
The word “flattr” is used as a verb, to indicate payments through the Flattr system - so when a user clicks a Flattr button and they are logged in to the Flattr site, they are said to be “flattring” the page they are on. Sunde said, “We want to encourage people to share money as well as content.”