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

• to run the program, for any purpose
• to study how the program works, and change it so it does your computing as you wish.  Access to the source code is a precondition for this. 
• to redistribute copies so you can help your neighbor, and
  
• to distribute copies of your modified versions to others.  Access to the source code is a precondition for this.
  

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.