Your question is Explain APIs to Non-Technical People. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
How would you describe what an API is to someone who is not technical?
Explain the idea in plain language, then connect the explanation to a concrete example such as the Salesforce REST API. Cover what an API provides, how a caller interacts with it, and why the API contract matters.
The interviewer is checking whether you can communicate a technical abstraction clearly without reducing it to an inaccurate analogy.