An Alias domain is a domain that is pointed to your main website.
For example, You purchase a basic plan for the domain "supermegawidgets.com" and you run an online business from that site. You also purchase "supermegawidgets.co.uk" and want both domains to show your main website.
To do this, simply login to your cPanel control panel and click the "Alias Domains" icon. Enter the domain you want to park in the first box and submit the form. Detailed Instructions on setting this up are available in this knowledge base.
Now, when anyone visits either domain in their browser, your main site is displayed.