From Google:
At Google I/O a few months ago, we called for “HTTPS everywhere” on the web.
We’ve also seen more and more webmasters adopting HTTPS (also known as HTTP over TLS, or Transport Layer Security), on their website, which is encouraging.
For these reasons, over the past few months we’ve been running tests taking into account whether sites use secure, encrypted connections as a signal in our search ranking algorithms. We’ve seen positive results, so we’re starting to use HTTPS as a ranking signal. For now it’s only a very lightweight signal — affecting fewer than 1% of global queries, and carrying less weight than other signals such as high-quality content — while we give webmasters time to switch to HTTPS. But over time, we may decide to strengthen it, because we’d like to encourage all website owners to switch from HTTP to HTTPS to keep everyone safe on the web.
Google has been talking about this for a while, but as of today, they’ve made it official. Though they say it’s a “lightweight” ranking signal, this will have a strong effect on the number of sites who go HTTPS. It won’t be long before sites using plain HTTP are an anomaly.
Not sure what HTTPS is and want to learn more? Check out this page on Wikipedia.