I've read somewhere that Emacs was important for the development of the idea of Free Software. How can that be if it's just a piece of software? Or is there more to it?
Emacs, in its incarnation as GNU Emacs, along with GCC, was one of the cornerstones of the GNU project, whose aim was to create a free Unix-like operating system. In 1985 GNU Emacs was licensed under the GNU Emacs License. The GNU Emacs License was the world's first copyleft license, and was eventually generalized to become what we know today as the GNU General Public License Version 1.
Encouraging others to adopt the same licensing practices meant closing off the escape hatch that had allowed privately owned versions of Emacs to emerge. To close that escape hatch, Stallman and his free software colleagues came up with a solution: users would be free to modify GNU Emacs just so long as they published their modifications. In addition, the resulting "derivative" works would also have carry the same GNU Emacs License.
The revolutionary nature of this final condition would take a while to sink in. At the time, Fischer says, he simply viewed the GNU Emacs License as a simple contract. It put a price tag on GNU Emacs' use. Instead of money, Stallman was charging users access to their own later modifications. That said, Fischer does remember the contract terms as unique.
"I think asking other people to accept the price was, if not unique, highly unusual at that time," he says.
I can offer a bit of history.
Once upon a time, there was TECO, a supercharged relation to what you might know, today, as 'ed'. Folks built a set of macros for teco that knew how to use the necessary escape sequences to do a WYSIWYG editor on a terminal. These macros grew into the first Emacs. It was not open source. It was not closed source. No one thought in those terms; it was just a thing at MIT.
Then, a friend of mine was inspired by this to build a version for Multics written in Lisp. Thus was born Multics Emacs, written (in part) in Lisp. Stallman, who was the primary author of the ITS TECO version, riffed off of it to create GNU Emacs, also in Lisp. GNU Emacs was a very early example of Stallman's FLOSS efforts; it was very widely used, because, well, it was useful. Because it was widely used, it was influential.