Google announced on October 26th that BrainRank is considered Google”s 3rd most important indicator in search results. RankBrain is Google’s name for a machine-learning artificial intelligence system that’s used to help process its search results. If a search query is something it does not understand, it decides what it seems most similar to and takes it as its own. So this machine is learning as it goes along.
See More info : https://en.wikipedia.org/wiki/RankBrain