What is WHMCS Integration?
WHMCS system is a 3rd party product that many hosts use to manage web hosting CRM, billing, helpdesk etc.
When you use this system on your website to take orders and to manage customers, it looks very different from rest of your website, because your website and WHMCS are two separate website areas.
WHMCS Integration means creating a custom template for your WHMCS area and match it with rest of the website. Usually it involves matching header/footer of the site for your WHMCS pages. That is the least you would like to do.
However there are further improvements possible like creating custom graphics for WHMCS items like in client dashboard page.