HTTPS is the secure version of HTTP, which stands for "Hypertext Transfer Protocol", the method used to transfer data between your computer and a website.  The 'S' at the end of HTTP stands for 'Secure', which means that all data communication between your computer and the website are encrypted and unviewable by other parties in transit.  Encryption is usually accomplished using one of two secure protocols called "Secure Sockets Layer" (SSL), or "Transport Layer Security" (TLS).