RankBrain launched in 2015 and is a way for Google to understand user queries better. Google has said you cannot optimize for it but you can work on making your site better. Gary Illyes from Google said basically making your content for users is accomplishing the same thing as making your content for RankBrain.
Gary Illyes said on Twitter "you optimize your content for users and thus for RankBrain." "That hasn't changed," he added.
you optimize your content for users and thus for rankbrain. that hasn't changed
— Gary "鯨理" Illyes (@methode) September 26, 2019
Back in 2016, a prominent Googler said Google doesn't fully understand RankBrain and another Googler said it is part of the top three ranking factors, which really didn't make much sense.
In any event, it sounds like if you work on making great content, you will at the same time, do a great job making RankBrain work for you.
Simple, right?
Forum discussion at Twitter.
Note: This story was pre-written before the Rosh Hashanah holiday. I am currently offline for the holiday and unable to respond to comments on this site, social, media or other platforms.