HTTP stands for HyperText TransferProtocol, 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.
You really don't need to know what it all stands for; the important thing is the letterS which makes the difference between HTTP and HTTPS. The S (big surprise) stands for "Secure". You probably didn't need me to tell you that, because you already knew it had something to do with security.
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 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.
There are some primary differences between http and https, however, beginning with the default port, which is 80 for http and 443 for https. Https works by transmitting normal http interactions through an encrypted system, so that in theory, the information cannot be accessed by any party other than the client and end server. There are two common types of encryption layers: Transport Layer Security (TLS) and Secure Sockets Layer (SSL), both of which encode the data records being exchanged.