UPDATE: As many of us suspected, Google has continued to apply RankBrain to increasing volumes of search queries - so many, in fact, that Google now says its AI processes every query Google handles, which has enormous implications for SEO.
As little as a year ago, RankBrain was reportedly handling approximately 15% of Google's total volume of search queries. Now, it's processing all of them. It's still too soon to say precisely what effect this will have on how you should approach SEO, but it's safe to assume that RankBrain will continue to focus on rewarding quality, relevant content. It's also worth noting that, according to Google, RankBrain itself is now the third-most important ranking signal in the larger Google algorithm, meaning that "optimizing" for RankBrain will likely dominate conversations in the SEO space for the foreseeable future.
To read more about the scope and potential of RankBrain and its impact on SEO, check out this excellent write-up at Search Engine Land.