Definition: HTTPS (Hyper-Text Transfer Protocol Secure) is the encrypted update to the HTTP protocol, adding a layer of security and reducing a website's susceptibility to attacks. Using HTTPS provides an assurance to customers and helps secure user data.
HTTPS creates a "secure connection," meaning three things:
As of the summer of 2015, Google starting using HTTPS as a ranking signal that will boost SEO results. Their motive was to automatically send users to a more secure connection during their Google searches, and they have even publicly called for the eventual Internet-wide use of HTTPS encryption.
For now, search ranking algorithms use HTTPS as a very lightweight signal, affecting less than 1% of searches and only acting as a sort of "tie breaker" for otherwise equal-quality content. However, Google has plans to strengthen the impact of HTTPS in the future, thus making "secure search" even more secure.