Search Suggestions (Autocomplete) are the queries an app store proposes as a user types in the search bar, ordered roughly by popularity — a free, real-time read on what people actually search for.
When a user starts typing, the App Store and Google Play complete the query with their most-searched matching terms. Because the order reflects real search demand, autocomplete is one of the cheapest keyword-research tools available: type a seed word, note what the store suggests, and you have a ranked list of phrases real users enter — including long-tail variants you'd never guess.
Suggestions also shape behavior, not just reveal it. A large share of searches are completed by tapping a suggestion rather than finishing the query, so appearing for the exact phrasing the store proposes is worth more than ranking for a near-synonym nobody types. Good keyword strategy starts by harvesting these suggestions, then prioritizing the ones with both demand and a realistic chance of ranking.
Example
Typing "workout" surfaces suggestions like "workout planner" and "workout at home"; a fitness app targets those exact phrases instead of the broad, brutally competitive "workout" on its own.