A grounded answer is tied to evidence the engine just retrieved; an ungrounded answer comes only from the model’s internal memory and is more prone to being outdated or wrong. Engines ground answers to improve accuracy and to provide citations, which is the mechanism behind tools like Perplexity and Google AI Overviews.
Grounding is the moment GEO can intervene. If your content is indexed, clearly written, and authoritative, it can become one of the sources the engine grounds on, which is how you earn a citation. Improving the quality and structure of your content directly improves your odds of being the grounding source.