RankBrain is a deep machine learning system developed by Google to better understand new and long-tail search queries and return more relevant search results. The use of RankBrain, as part of the core algorithm, was confirmed by Google on Oct. 26, 2015.
Using artificial intelligence, RankBrain embedded written language into mathematical entities called word vectors to learn word associations and better understand long-tail queries and queries that Google had never seen before.
There are no SEO best practices for RankBrain specifically.
RankBrain helps Google understand and interpret search queries and return relevant results—even if they don’t have the same words. So the best thing to do is write your content conversationally and naturally.
According to Google’s Gary Illyes at Big Digital Adelaide 2016:
Optimizing for RankBrain is actually super easy, and it is something we’ve probably been saying for fifteen years now, is – and the recommendation is – to write in natural language. Try to write content that sounds human. If you try to write like a machine then RankBrain will just get confused and probably just pushes you back. […] If it sounds conversational, if it sounds like natural language that we would use in your day-to-day life, then sure, you are optimized for RankBrain.