from another comment I made on licenses:

They are needed to tell users and developers what they can do with the project and whether they can change the source code, redistribute it, etc. Having no license by default means others can’t look at your code or modify it in any way, as the terms on how to do so are not defined!

There are several licenses that are used for open-source projects. Generally, they are grouped as either permissive licenses (like MIT) or copyleft/protective licenses (like GPLv3). In a nutshell, permissive licenses gives the developer (or, in the case of commercial use of open-source code, the company) more freedom as the code can be used in any kind of project, including proprietary ones. In contrast, copyleft licenses aim to give users more freedom by ensuring that the code can only be used in projects that also use an open-source license.

There are other elements to licenses too, like how code used should be attributed, whether you are allowed to fork the project, additional copyleft restrictions for SaaS applications (see AGPLv3), loosening of copyleft restrictions (see LGPLv3), etc.