It calculates a percentage which shows how many of the requests for this page, are being served by a Varnish Cache Proxy!The Varnish Cache Efficiency Extension is useful, if you want to check how many of the request
necessary for the current page are delivered by a Varnish Cache Proxy. The shown number is a
percentage value which says how many percent of the requests are delivered out of the Varnish
Cache.
To function the Varnish Cache Proxy needs to be returning the "X-Varnish" Header.