A domain alias is an alternative name for your domain.  For example, perhaps you have your site all up and running on myfabwebsite.co.uk but you also own the domain myfabwebsite.com.  If you add it as an alias, and point the DNS to the server, traffic to myfabwebsite.com will hit myfabwebsite.co.uk. It's not limited to a different .co.uk/.com, if you also owned mysmashingwebsite.co.uk, that could go in there too.  You'll need to add the wwws for each one too as those are classed as different domains.

From the dashboard, click on the cog icon next to your website to manage the hosting for that website. 

Click on Aliases & Redirects

Click Add Domain

Type in the desired domain name, to use the domain as an alias leave the Redirect box unchecked and click Create Domain

DNS for the alias domain will also need to be pointing at your server, see this guide