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. Hence the “no warranty” point above.