A Wildcard SSL certificate is used to secure unlimited sub-domains within the same level that share a common base domain.
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 the bare domain mydomain.com.
Comodo (now Sectigo) certificates issued for *.sub.mydomain.com will also secure sub.mydomain.com.