Answer Description
A wildcard domain applies to the domain and any subdomains. For example a certificate for *.google.com could be used on mail.google.com, photos.google.com, etc. This way it is not necessary to create and manage individual certificates for all of these sub domains.
Wikipedia
In computer networking, a wildcard certificate is a public key certificate which can be used with multiple sub-domains of a domain The principal use is for securing web sites with HTTPS, but there are also applications in many other fields Compared with conventional certificates, a wildcard certificate can be cheaper and more convenient than a certificate for each sub-domain Multi-domain wildcard certificates further simplify the complexity and reduce costs by securing multiple domain and their sub-domains
Wildcard_certificate - Wikipedia, the free encyclopedia