The S stands for "Secure"..
This means that the website is talking to your browser using the regular unsecured language. In other words, it is possible for someone to "eavesdrop" on your computer's conversation with the Website. If you fill out a form on the website, someone might see the information you send to that site.
This is why you never ever enter your credit card number in an http website...
but if the web address begins with https://, that means your computer is talking to the website in a Secure code that no one can eavesdrop on. You understand why this is so important, right?