SSL, or secure socket layer, is a security feature that allows all communication between a browser and server to be encrypted. You will know a page is encrypted when the "lock" appears at the bottom of your browser window. A secure site is also indicated by the fact that the beginning of the URL you are viewing changes from http:// to https://.
Sites that need to transfer sensitive information such as passwords and credit card information from the browser to the server need to have SSL enabled.