Free Software is software that has been released under a licence that gives the user of the software four essential freedoms:
- the freedom to run the software as he or she sees fit
- the freedom to study the software, and the software's source code
- the freedom to modify the software and its source code, as the user sees fit
- the freedom to re-distribute the software and its source code, either in its original form, or in a form modified by the user
You can learn more about Free Software here.