Above video by inventor of react-router will make you to understand what it is and how good it really works..
Basically, it remembers your url each time of your navigation using transitions. Say, you are in one page where you were trying to see your friend profile. Then immediately after clicking your friend profile, it asks for login authentication. Now after logging in, it should redirect to your friend profile instead of home page. This is called transition(in simple words url memory). If you go through https://github.com/rackt/react-router code, you will understand how that works clearly
There are different states in transitions like abort etc. We will go through those things later.
We used mixins in transitions to achieve the functionality. Mixins are to Pass functionality to multiple components. Looking for more updates on that?? then watch here