Debian Free Software Guidelines
The Debian Free Software Guidelines (DFSG) defines what Debian believes is Free Software.
It is a kind of agreement with the users of Debian as to what they can expect to do with the software that Debian packages for them.
For package maintainers it's most useful function is to define
what can go in to
main, the Debian distribution. If the license
your package is under doesn't meet the DFSG then it cannot be
main. (See Licenses for
more on this).