If you are new to Varnish, here is what Wikipedia says about ti:

Varnish is an HTTP accelerator designed for content-heavy dynamic web sites. In contrast to other HTTP accelerators, such as Squid, which began life as a client-side cache, or Apache, which is primarily an origin server, Varnish was designed from the ground up as an HTTP accelerator.

You can also read Varnish Cache homepage, and here is what I've written about varnish.

Well today I want to show you how to purge all the cache without the need to restart the daemon.

varnishadm -T url.purge .

That will flush the cache.