Varnish is an important piece of software for any Magento website because it significantly improves performance. Varnish accomplishes this by caching pages in memory and serving to visitors, which is extremely fast. Otherwise, Magento would have to build the pages in your website from "scratch" whenever someone visits a page, which can be excruciatingly slow. You can use the following validator to test if your Magento website is running Varnish cache:
Simply enter in your website and click Check It, what could be easier?
If you would like to learn more about Varnish and how it can help your Magento website, visit their website: