Varnish is a caching server that stores copies of images (.png, .jpg, etc), documents (.pdf, .txt, etc), and pages. It receives all traffic for your website and checks if it has a cached version of the requested content before requesting it from your site.
Images and documents will be stored in the Varnish cache for 5 minutes before your website is checked again for the current version. This means if you replace a document with a new version and keep the same filename, visitors may still see the old version for up to 5 minutes. Likewise, if you delete a document, the document may still be visible for up to 5 minutes.
Pages are also kept in the Varnish cache. A copy of a page will remain in the cache for up to 3 hours. However, unlike for documents and images, your Drupal site will automatically clear this cache when updates are made. All visitors should see content changes you made within 1 minute after the update.
Note that when you are logged into your site, the Varnish cache is completely bypassed.