SSL means Secure Socket Layer.
SSL is used to provide secure communications between you and the SSL enabled website, so that information you provide to the website is not sent over the Internet in plain text, but instead encrypted so that information you send or receive can not be intercepted and de-crypted by a 3rd party.
SSL is also used to validate the identity of the website holder, giving your customers the peace of mind. To see if the website is SSL enabled look for the padlock symbol in your browser and the HTTPS prepend of the URL.
