How It Works?
SSL (Secure Sockets Layer) issued by a trusted authority such as RapidSSL or GeoTrust is installed on your domain's hostname i.e. secure.domain.com. When there is a client of yours that browse over https://secure.domain.com the connection between the browser and the server is encrypted with 128, 256 bit key. Any data send between your client's browser and the server is encrypted before sending and decrypted after received on the server. In that way information transmitted over the Internet is encrypted and signed as secured by this trusted authority. Browser when sending data over https:// checks the validity of the SSL Certificate with the issuance authority. SSL technology protects your Web site and makes it easy for your Web site visitors to trust you.
What is Wildcard SSL?
Regular SSL Certificates are issued and signed to secure single hostname of your domain. In other words you can use it only for one particular subdomain e.g. www.domain.com or secure.domain.com. If you want to have encrypted secure connection over several subdomains you need Wildcard SSL Certificate. It can be used to secure multiple (as much as you have) subdomains on a single domain name.