Free software is software that gives users the four essential freedoms:

• to run the program, to study and change the program's source code,
• to make and distribute exact copies, and to make and distribute
• copies of modified versions.

For more information, see the definition of Free Software on the Free Software Foundation's (FSF) website.  The FSF promotes the development and use of free software, particularly the GNU operating system, used widely in its GNU/Linux variant.