The goal of the add-on is to encrypt all of your Web browsing traffice regardless if the visiting site has implemented SSL or not.
"This Firefox extension was inspired by the launch of Google's encrypted search option. We wanted a way to ensure that every search our browsers sent was encrypted," according to the EFF announcement.
It doesn't work with every site, but it's a beta, and you can add your own rules to encrypt other sites too.
HTTPS Everywhere is open source and released under the GPL v2 license.