The wildcard applies to the PositiveSSL DV, Domain, and Organisation SSL certificates. The certificates are issued for the domain name specified in the initial application. If you buy a wildcard with one of these three certificates, it means that the common name specified is *.yourdomainname.com. You can then use the certificate on an unlimited number of subdomains and servers. You also have the option to add in the future.
If you need to secure multiple subdomains on a single domain name, you can buy a wildcard. For a wildcard SSL certificate, a common name of *.yourdomainname.com would secure www.yourdomainname.com, mail.yourdomainname.com, secure.yourdomainname.com, etc...