If you want to learn Varnish Thijs Feryn wrote a book for you. It's free to download until 7th March 2017.
Getting Strated with Varnish Cache is a technical book about the Varnish caching technology. Varnish is a so-called reverse caching proxy that acts as an intermediary between the browser and the webserver. Varnish stores HTTP responses and serves them to the browser, without accessing the backend for every request. This causes a massive speed increase.