Varnish is a HTTP/web page accelerator. We use Varnish both as a load balancer and to speed up page loads by having varnish answer requests, instead of storage servers (which answer slower). Basically Varnish makes a web page faster.
Sometimes it is useful to temporarily disable the varnish cache, for example when editing your website. You can do this by adding a .htaccess file with the following code to the directory that you are editing.
Header add "Cache-Control" "no-cache"