“Free software” means software that respects the freedom of the people who use it. As the Free Software Foundation’s definition page stresses, “free” here refers to liberty, not price: the point is “free speech,” not “free beer.” A program qualifies as free software when it grants its users four essential freedoms.
The FSF numbers these freedoms starting from zero. Freedom 0 is “the freedom to run the program as you wish, for any purpose.” Freedom 1 is “the freedom to study how the program works, and change it so it does your computing as you wish,” which requires access to the source code. Freedom 2 is “the freedom to redistribute copies so you can help others.” Freedom 3 is “the freedom to distribute copies of your modified versions to others.”
The definition distinguishes free software from “open source.” The FSF page notes that although nearly all open source programs are in fact free software, open source rests on “a very different philosophy” with different values. The free software framing puts user freedom, rather than the practical convenience of shared development, at the center.
These four freedoms are the practical test the movement applies to any program. They underpin the GNU Project’s licensing, including copyleft and the GNU General Public License, which are designed to secure these freedoms rather than merely declare them.