Your site will ask your users to allow to show notifications. Since allowed, your website will serve notifications about friendship requests, messages, regular notifications and more! Of course yoursiteURL.com will be replaced by your own website's URL.
You'll only need to install the plugin and it will work right away. No boring and complicated guides. There is a very basic set of preferences to allow you to set notifications frequency, limit how much of them will show at the same time and how long notification will show on screen till it disappears.