Query fan-out is when an AI search system breaks a single user question into several underlying searches, runs them, and synthesizes the results into one answer. It lets the system cover multiple facets of a question rather than relying on a single search.
Fan-out means your content can be retrieved for sub-questions a user never typed word for word, which rewards covering a topic thoroughly rather than targeting one exact phrase. It is a feature of modern retrieval-based AI search.