Search Match is an Apple Ads feature that automatically matches search results ads to relevant App Store queries using the app's metadata and data on similar apps, so advertisers needn't list every keyword.
Search Match removes the need to anticipate every query. When it's on, Apple Ads draws on the metadata in your App Store listing, information about similar apps in the same genre and other search data to decide which searches your ad can appear for. It's on by default in Manage Bids search results campaigns, where it can be switched off per ad group, and it's required for the automatic ad group in Maximize Conversions campaigns. It applies only to search results ads; the other Apple Ads placements don't use keywords at all. Reporting shows a match source for each impression: Search Match or a bidded keyword.
For ASO teams, Search Match doubles as a rough readout of how Apple interprets the listing. Because it leans partly on your metadata, the queries it matches hint at which themes Apple associates with your app, and in our experience obvious gaps suggest the name or subtitle could be clearer. Apple's campaign-structure guidance gives it an ad group of its own inside a discovery campaign, with Search Match on and no keywords, and adds every keyword already bid on elsewhere as an exact-match negative. Search terms that perform there move into exact-match campaigns, and onto the metadata shortlist.
Example
A recipe app runs a discovery campaign with Search Match on. After a month, the Search Terms report shows steady installs from "meal prep planner", a phrase absent from its metadata, so the team moves it into an exact-match campaign and adds it to the keyword field for the next release.