What does the "Maven Surefire plugin" means when it test your build?

I am working on understanding Maven and I'm learning about building your Java app with it. So when I do a :

maven package

It does build my jar as expected but I see in the output console that Maven does build tests (it always say that the test a run and there are no failure). I researched on the web about that and learned that Maven use a plugin called Maven Surefire. But I can't understand what does that plugin do to my code, what does the tests "means" ? What does the tests do with my code and how it works behind the console ?