HTTP stands for Hyper Text Transfer Ptotocol, which is a protocol for information to be passed back and forth between the Internet and your machine (computer).
If you are visiting a website or webpage, and the web address begins with http://, it means that the website is using the regular 'unsecured' language. In other words, it is possible for someone to 'eavesdrop' on your computer's conversation with the website.
HTTPS - The 'S' stands for 'Secure' Hyper Text Transfer Protocol.
If the website address begins with https://, that basically means that your computer is talking to the website in a secure code that no one can eavesdrop on.
Hence, if a website asks you to enter your credit card information or bank account details, you should automatically look to see if the web address begins with https://. If it doesn't, DO NOT PROVIDE any sensitive information like a credit card number.