Varnish cache is a reverse proxy caching that increase the web site speed by serving the content on behalf of the webserver.
Follow the steps to enable Varnish cache:
Step 3: Click on the ‘Sites’ to open the site list
A confirmation box will be displayed. Click on Continue button and the Varnish status will change.
The toolbox offers other tools too. You can enable Redis cache from here, or flush all the cache of the website. Though, make sure to take a backup before making any significant changes to the site.