The skeleton directory, is a feature available in Web Host Manager (WHM), the control panel that comes with reseller account on cPanel servers. This directory is used as a template, or skeleton, for new cPanel accounts. Anything placed in the skeleton directory of the reseller account is automatically copied to the home directory of any new cPanel account created by that reseller.
For example if, in the skeleton directory, you create a /public_html directory include an index.html file, each new cPanel account you create will have a copy of that index.html in their public_html directory.
To know your skeleton directory path:
Login to WHM
In the Account Functions menu, click on the “Skeleton Directory” link.
The path to your skeleton directory will be displayed, usually its something like,
where “username” is your main reseller account.
You could put information telling your clients how to begin modifying their cPanel account settings, uploading their own web pages, etc. Or in an index.php file, you could include more dynamic contents. Or you can welcome your clients by putting welcome texts or can put heading like that ******* site is proudly hosted by ********. Some include a page with steps for clients on how to begin configuring their accounts.
So when your client opens the website, he will see a default index page ie the page placed in skeleton directory. its good feature to add a default index page so that there will not be directory listing show for newly created accounts.