To use your own URL for the Custom Domain.
Examples:
weddings.yourdomain.com (this is a subdomain)
yourdomain.com (this is a main url)
Using your own URL will improve your site's SEO and keeps your branding consistent.
You will need to register and use your own domain name.
It takes a few steps to set this up the first time.
To set up your Custom Domain start here: Setting up your Custom Domain