Free Software Movement is political movement to free software users from corporate control. Officially it began on October 4th, 1985. It was two years after he publicly announced that his intention to create a free Unix-like operating system called GNU.
To give software users full rights and control Stallman created a license based on the idea of Copyleft. You can read more about it here. Based on that he created 4 laws for softwares. They are following:
1 The freedom to run the program as you wish, for any purpose (freedom 0).
2 The freedom to study how the program works, and change it so it does your computing as you wish (freedom 1). Access to the source code is a precondition for this.
3 The freedom to redistribute copies so you can help others (freedom 2).
4 The freedom to distribute copies of your modified versions to others (freedom 3). By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this.
These all together is called Gnu General Public License (GPL). If any software which is released under this is called Free Software.