Link from one website to another. It’s called a "Back" link because it links back to the original source of something.
In the context of b2evolution, "Back-Link" is the term some use to define links from your b2evolution-powered website to b2evolution.net.
Giving credit to the b2evolution project is not a requirement for using the software, but forum users are not required to help you either. Users who are willing to have that link on their site show support for the project, help advertise b2evolution and improve the project’s search engine rankings. Supportive users are more likely to get help in the forums.