Let’s face it, there is a bunch of information out there regarding RankBrain a machine learning (Artificial Intelligence) part of Google’s ranking system and it’s confusing. Here is the most simple explanation. One Suitable for a client to understand.
If Google has not seen that specific search before. Rankbrain goes though as much information as it can to try to figure out your intent for that search then give you those results.
That could mean your previous search history, what others searched recently, trending searches and a few (actually many) other factors. It then gives you what it feels you were looking for.
It continues to learn from you and everyone else. Overtime it will get smarter as it learns.