HTTP stands for Hyper Text Transfer Protocol.
The "s" stands for "secure".
If you visit a website or webpage, and look at the address in the web browser, it will likely begin with the following: http://. This means that the website is talking to your browser using the regular 'unsecure' 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 basically means your computer is talking to the website in a secure code that no one can eavesdrop on.
If a website ever asks you to enter your credit card information, you should automatically look to see if the web address begins with https://.
If it doesn't, you should NEVER enter sensitive information ... such as a credit card number.