A Wildcard SSL certificate is used to secure unlimited sub-domains that share a common base domain.
But the domain itself won't be protected. For example: A Wildcard certificate issued to *.mydomain.com can be used to secure the following domains:
- login.mydomain.com
- secure.mydomain.com
- support.mydomain.com
- www.mydomain.com etc.
Along with multiple subdomains, a Wildcard SSL will also secure naked mydomain.com