The four essential freedoms are the operative core of the Free Software Definition. The Free Software Foundation states that “a program is free software if the program’s users have the four essential freedoms,” and that these freedoms must be available to everyone who has a copy. They are numbered starting from zero, a convention that nods to programmers’ habit of counting from zero and signals that the right to run the program at all is the most basic of the set.
Freedom 0 is “the freedom to run the program as you wish, for any purpose.” The gnu.org text emphasizes that this means any kind of person or organization may use it on any kind of system for any kind of job, “without being required to communicate about it with the developer or any other specific entity.” There may be no restriction on the purposes for which the software is used.
Freedom 1 is “the freedom to study how the program works, and change it so it does your computing as you wish.” The definition immediately notes that “access to the source code is a precondition for this,” because changing a program is not practical without being able to read and modify its source. Freedom 1 turns the user from a passive consumer into someone who can adapt the tool to their own needs.
Freedom 2 is “the freedom to redistribute copies so you can help your neighbor,” and Freedom 3 is “the freedom to distribute copies of your modified versions to others.” The gnu.org text explains that by exercising freedom 3 “you can give the whole community a chance to benefit from your changes,” and that, as with freedom 1, “access to the source code is a precondition” for distributing useful modified versions. Together freedoms 2 and 3 cover sharing exact copies and sharing improved copies.
The numbering and wording have been stable for decades even as the surrounding explanatory text was refined. A program that grants all four freedoms is free software; withholding any one of them makes the program non-free in that respect. Because the GNU General Public License and the broader copyleft strategy exist precisely to secure these freedoms for every downstream user, the four freedoms are the yardstick against which the FSF measures every license it evaluates.