Words and phrases connected by meaning to a subject, helping explain it accurately and naturally.
Semantically related terms are words or phrases connected through meaning. Some are synonyms, but many describe different parts of the same subject. On a page about bicycle maintenance, a chain, lubricant and a drivetrain can be related concepts without being interchangeable words.
These terms are useful when they help explain the actual topic. A writer should choose them because the audience needs the concept, not because a tool says every competing page contains it. A technically related subject can still be irrelevant to the specific question a page is answering.
Google’s language systems can connect queries and content without exact repetition of every phrase. That supports writing naturally, but it does not turn a vocabulary list into a ranking formula. Avoid calling a tool’s suggested terms mandatory or implying that mentioning them proves subject expertise.
Takeaway: Use related concepts to make an explanation complete and precise. Include only the terms you can explain accurately and that help the reader understand this particular page.
Example
A guide to choosing a dining table may discuss seating capacity, clearance and extension leaves. A page about refinishing the same table needs different concepts, such as surface preparation and the existing finish. Copying one universal furniture keyword list into both pages would blur their different purposes.
What to check
If a writing tool suggests a term, ask what useful explanation it would add. Research unfamiliar concepts before using them, and do not insert technical language just to improve a score. A synonym that changes the meaning is not a harmless substitute.
For more detail, see SEO Starter Guide.