Social signals are the likes, shares, votes, pins, or views people place on Facebook, Twitter, LinkedIn or other social media sites that filter out to the various search engines.
A search engine's primary aim is to provide high-quality, relevant content to searchers. Search engines, such as Google, judge quality in large part by how frequently a URL is seen throughout the internet, and as more web communication has moved to social media, Google and other search providers have integrated social signals as a ranking signal.