HTTP stands for « Hypertext Transfer Protocol ».
HTTP is a client–server communication protocol invented by Tim Berners-Lee. It defines and enables access to the web pages we call « the internet ».
It is an evolution of FTP (File Transfer Protocol), which transferred files without considering their format. Information was sent, but it was up to the receiver to interpret images, sounds, text and so on.
HTTP can read the data format (via MIME). Combined with the HTML language (designed to write hypertext documents, now called « web pages ») and web addresses (commonly known as URLs), these three inventions laid the foundation of the World Wide Web — the famous www. The internet for the general public was born in 1990 from this combination of innovations.
What about HTTPS? It is an extension of HTTP, where the S stands for « secured »: it allows encrypted data exchange and makes interception or tampering impossible.