Query Fan-Out is the mechanism by which a generative AI breaks a complex question down into several smaller sub-questions and searches for answers to each one separately. The AI then synthesizes all the partial answers into a single, complete response.
This matters for GEO because it means your content does not have to answer only the user's main question. If your article answers one of the sub-questions generated by the fan-out well, it can be cited in the final answer. This greatly expands your opportunities to be referenced.
In practice, the implication for content creation is clear: beyond pillar articles that cover a topic broadly, it is important to have satellite pieces that answer specific, granular questions. This cluster model (pillar + satellites) was already an SEO best practice, and with query fan-out it has become important for GEO as well.