Wildcard DNS allows all subdomains under a domain to show the same content as the main website.
Also, it allows all subdomains to share the same set of files, which you may need for your multi-site web application.
So, if anyone types index.your-site-name.com or blog.your-site-name.com, it will be forwarded to your-site-name.com.
You can find the Wildcard DNS feature in the Hosted Domain section. Just click on the Host Domain button on the top right and in the popup box select the Wildcard Domain option at the bottom of the list of settings.