The Add Website feature allows you to add additional domains to your account that will act as a separate website :)
To create a new add-on domain, head over to your hosting account's hPanel and search for the Add Website menu in Domains section.
In the Add New Add-on Domain section choose your domain type - you can either use your own registered domain or get a free subdomain from Hostinger. Type in a password and once you are done, click Add.
NOTE: If you want to use your own domain as an add-on domain, make sure it's pointing to our nameservers.