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 domains and their sub-domains.