Search Volume (Popularity Score) is an estimate of how often users search a keyword — Apple expresses it as a 5–100 popularity score, and it tells you the traffic a top ranking could deliver.
Search volume is the demand side of keyword research: a perfectly ranked keyword that nobody searches sends no installs. Apple doesn't publish raw query counts; instead, Apple Search Ads exposes a popularity score from roughly 5 to 100, and third-party ASO tools model their own volume estimates from it. Because the scale is non-linear, the gap between a score of 40 and 60 is far larger than the numbers suggest.
Volume only becomes useful next to difficulty and relevance. High-volume head terms are tempting but usually too competitive for a young app to win; mid-volume long-tail terms often deliver more actual installs because you can realistically reach the first page. The goal is the highest volume you can credibly rank for, not the highest volume that exists.
Example
"workout" has a high popularity score but is hard to win; "home workout no equipment" has lower volume but converts better and is reachable, so it nets more real installs for a new fitness app. Read volume next to difficulty and relevance; the best target is the highest volume you can realistically rank for.