When we send data over the Internet, it is encrypted. HT stands for hypertext, and the next T stands for transfer. That is, the textual information we are sending is being transferred to the recipient as hypertext. The following letter, P, stands for Protocol.
This is a system of rules that we apply so that all using it know how things are to be done. The last letter - S - stands for secure. URLs used to be merely HTTP and then the WWW. Now they are HTTPS in recognition of the ever-increasing invasion by unwanted persons, hackers, and therefore the increasing need for more and more security when sending information over the Internet.