It's one of several options for software licenses a developer or team can use when distributing a piece of software to help ensure that it and its derivates stay free and open-source.
Not sure whether to be dismayed or excited at the fact that this is a question on Hacker News, but those who are new to it; definitely worth doing your homework.
It's the legal jiu-jitsu that has enabled Linux and countless other "Open Source" projects to remain free by preventing people from taking the code for free but then locking it back down.
