You can either install an SSL certificate for yourdomain.com or for any subdomain like www.yourdomain.com, secure.yourdomain.com and others. In case you want to secure all subdomains of your domain(only one domain, not all domains in cPanel), you will need to get a WildCard certificate. With Namecheap, you can purchase PositiveSSL WildCard and EssentialSSL WildCard.

The procedure of activation and installation of a WildCard certificate is the same as for an ordinary one. You can find the instructions here.