Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Before "open source", before free software, there was software in the public domain. You could say that software in the public domain was truly free. The code was "open source" and the user had the right to take it, break it, appropriate it, re-use it, package it, sell it, re-brand and license it, or do what you will with it.
The problem with software in the public domain, was that, more often than not, any changes to the software didn't come back to the developer in any usable form, and modifications didn't revert to the original maintainer of the code. In some cases the software was appropriated and relicensed by the user. In other words there was no guarantee that the software would remain free, or would grow.
The solution to this problem was the GPL. Not only did the GPL protect free software and keep it free, but it opened the door for a community to form around the software in the knowledge that contributions were fed back into the original body of code. The often criticised "viral" nature of copyleft and the GPL was the instrument that fostered participation and feedback from the community, and provided the framework for the later success of free and "open source" software.