HTTPS is an encrypted protocol for transmitting HTML pages.
HTTPS encrypts the standard HTTP communications using the Transport Layer Security (TLS) protocol. There are various levels of encryption with 256 bits being one of the most secure encryption methods.
Google announced in August 2014 that their ranking algorithm would use HTTPS as one of the signals it uses to determine a web page's rank so all other factors being equal a website with HTTPS will rank higher than one that is not. The main advantage of HTTPS over the HTTP protocol is that can protect users from man-in-the-middle and eavesdropping attacks which can compromise private and financial information.
More information from Google about HTTPS as a rank signal