HTTP stands for HyperText Transport Protocol, which is just a fancy way of saying it's a protocol (a language, in a manner of speaking) for information to be passed back and forth between web servers and clients.
The important thing is the letter S which makes the difference between HTTP and HTTPS.
The S (big surprise) 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.
You understand why this is so important, right?
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, there's no way you're going to enter sensitive information like a credit card number!