What's "Free Software"?

"Free Software", or sometimes "free and open source software", is a program that has the source-code (the programming instructions) openly published and available for anyone to use to make new programs. Firefox is a famous example of free software.

Do I have to do mess with my firewall or router to make Dijjer work better?

No. Lots of p2p apps (including Bittorrent) make you forward ports to get good performance. But Dijjer uses state-of-the-art "NAT2NAT" techniques to get the most out of your internet connection without any reconfiguration.

So downloads happen in my browser, but they come from a p2p network? How's that?

Dijjer runs something like a fake website on your computer, accessible only to you. So Dijjer links point your browser to this address, and your browser thinks it's getting the file from the web when really it's coming from Dijjer. Dijjer, in turn, gets parts of the file from the original website and most of it from other Dijjer users.