Hey,
it depends what kind of restrictions are you talking about.
A program is free software when guarantees:
- The freedom to run the program, for any purpose (freedom 0).
- 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.
- The freedom to redistribute copies so you can help your neighbor (freedom 2).
- 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.
A GNU license guarantee all those core freedoms to all users. The restriction is that users have to apply the same license to anything they do with or from your software. They can also sell what they do with your software, but must ensure the same license is applied.
MIT license, for instance, guarantees core freedoms (please, correct me if wrong), but allows users to change the license of their derivative works.