What Varnish does (it can be bad understanding)

Varnish is a webserver software that you place between your website visitor and Apache.
Varnish caching static elements of a webpages (images, js, css..) in RAM and so it can serve your website visitors repeated identic request with extreme speed instead of reading files from a disk via Apache.
Some elements are not cached so Varnish forwards the requests to the Apache.

Varnish Install tutorials

There are several tutorials on how to apply Varnish cache to the server with CentOS and WHM/cPanel. These looks good

https://www.varnish-cache.org/installation/redhat - Universal Install of Varnish alone RHEL5,6
http://crybit.com/how-to-enable-varnish-in-cpanel-server/ - RHEL 5 (but very good tutorial)
http://www.cpanelkb.net/how-to-install-varnish-cache/ - for RHEL 6 (Centos 6)
http://support.qualityunit.com/496090-How-to-install-Varnish-with-CPanel-and-CentOS-to-cache-static-content-on-server - for RHEL 5 (Centos 5)
https://www.digitalocean.com/community/articles/how-to-install-and-configure-varnish-with-apache-on-ubuntu-12-04--3 - Ubuntu 12.04 Install