On occasion, you may see code that has been released under an “MIT license”. This license entails three basic things:
- You can do whatever you want with the code, including using it in any type of project, and re-publishing it under a different license.
- There is no warranty.
- If you redistribute most of the code together, you need to acknowledge where it’s from and that it was MIT-licensed by the original author.
That’s it, to my understanding. A clear and slightly more detailed explanation can be found at tl;dr legal.
As I understand, the MIT license (and similar licenses, such as BSD-type licenses) are called “permissive” licenses because they are designed to be as permissive to the user as possible — their main purposes include letting the user do anything with the code, while making sure that the author is protected from being sued if the code doesn’t work correctly.