What it is
Query fan-out is when an AI search system takes a single question and fans it out into multiple background searches. Ask a broad question and the engine might silently search for the definition, the comparison, the pricing, and the reviews, then weave the answers together. You see one tidy response; the engine ran a dozen mini-searches to build it.
Why it matters
Fan-out changes who gets cited. Instead of one page winning one query, many pages can each win a slice of a larger answer. That rewards sites with thorough, well-structured coverage of a topic, because they are likely to be the best source for several of those sub-questions at once. It is a direct reason topical depth now beats a single thin page.
What to do
Cover topics completely rather than chasing one keyword per page. Anticipate the follow-up questions around a subject and answer each one clearly, so your content can be pulled into multiple branches of a fanned-out query.