An SSL Certificate is used to encrypt the connection between your website and the web browser of viewers of your website. This is essential when your website asks for sensitive information from your viewers, such as a password or credit card number, and it also gives viewers a sense of security knowing their transaction with your website is secure.
Most SSL Certificates are tied to a single domain name including any subdomain (eg. www.yoursite.com or secure.yoursite.com), while Wildcard certificates are tied to a single domain name (eg. yoursite.com) but allow the use of as many subdomains as you like (eg. secure.yoursite.com, shop.yoursite.com, etc).