Google’s machine learning system introduced in 2015 to understand relationships between words and concepts, especially for unfamiliar queries.
RankBrain is a Google AI system associated with understanding relationships between words and concepts. Its significance for content planning is that relevance is not limited to mechanically repeating the exact wording of a query. That does not mean the system exposes a public list of required synonyms or a score a website owner can directly optimize.
A useful page should explain the subject in language appropriate to its audience. Different customers may describe the same problem differently, but adding every possible phrasing can make the page awkward and less clear. Include alternate terms when they help understanding, not because a tool claims an arbitrary number is needed for RankBrain.
Be cautious about attributing individual results to a named system. A page appearing for an unexpected query is an observation about search behavior, not proof that you have identified the mechanism responsible. Use such observations to learn how readers describe their needs and improve genuinely missing explanations.
Takeaway: Write clear explanations of the problem and solution, using natural terminology customers understand. Do not buy or create a supposed RankBrain keyword-density formula.
Example
A bicycle shop explains a rubbing disc brake and the checks involved in assessing it. Customers may search using everyday descriptions rather than the exact component name. The page connects those descriptions to accurate service information without repeating a large block of near-identical phrases.
What to check
Review unfamiliar queries for actual relevance before expanding content. A loosely related search appearance does not mean the business should add an unrelated service or promise a result it cannot deliver.
For more detail, see Google ranking systems guide.