With a Wildcard SSL Certificate you can secure unlimited subdomains of a domain with a single certificate.
This means that you can provide SSL Certificates "* .mydomain.com" with a single click.
It will save you a lot of effort and cost when using multiple subdomains compared to using multiple individual certificates.
Please note that no Wildcard Certificates are available if you use our free Let's Encrypt Certificates. In that case you have to activate SSL for each domain/subdomain separately.