Rankbrain is a Google algorithm used to understand search queries and sort search results using machine learning.
RankBrain adapts the results depending on the keyword type reassessing the importance of domain authority & page authority, backlinks, content freshness & length. RankBrain also assesses how users interact with search results and adapts the algorithm depending on how they interact with them.
RankBrain uses keywords it has already dealt with to understand searches never yet seen before (which represent over 15% of all searches) to transform keywords into concepts and then analyse user's response to the results (CTR, Dwell time, Bounce rate, number of results clicked on).
For keyword targeting, this means you should be targeting medium tail keywords that will then in turn rank for many similar related other keywords.