Based on site: PlumRocket
In this guide, we will show you how to install and configure the Varnish cache for the Apache webserver with SSL termination. One limitation of Varnish Cache is that it is designed to accelerate HTTP, not the secure HTTPS protocol. Therefore, some workarounds must be performed to allow Varnish to work on SSL enabled website.
SSL/TLS Termination is the process of decrypting SSL-encrypted traffic. In our case, Apache acts like a proxy server and intermediary point between the client and Varnish and is used to convert HTTPS to HTTP.