Licenses are how we tell other people how to treat our code. Usually you'll have a license file in your project so it stays with your code. There are a couple of broad categories of open source licenses.
Permissive licenses are less restrictive. In general, they don't require anyone using your code to make their project open source. However, they usually have to keep your license with your code. The MIT License is a very simple and popular permissive license.
Copyleft licenses require any project using your code to also be open source. GPLv3 is a good example of this.
Other popular licenses are Apache, which grants patent rights to users, and BSD 3-Clause, which prevents people from using your/your organization's name to endorse their project. Keep in mind if you don't list a license, then the code is considered copyrighted by you, and others can't use it.
The best way to choose a license, is to think about what your worried about (such as your code being used for proprietary project), and choose a license that protects against that.