Search Ads have been in beta testing since the feature was announced, and developers were previously able to participate in a beta testing program to provide ads that were visible to App Store users.
With Search Ads, developers can bid to have their app listed as the top result when certain keywords are used, improving app discoverability. Ads are available through an auction system, with one ad displayed to customers on each search result page. Ads are clearly marked and ad content is the same content that's available on the App Store app description pages as it uses App Store metadata and imagery.
According to Apple's Phil Schiller, Apple "thought about how to carefully" do search ads in a way that "customers will be happy with." Apple also believes the ad auction system is fair to developers, including indie developers.
Developers do not need to pay a minimum amount to use Search Ads, so it's possible to buy ads for a small sum of money. Apple prices Search Ads based on a cost-per-tap model, so developers only pay when a user taps on an ad. Apple displays ads based on a combination of bid amount and an app's relevance to the search query, with ad matching done automatically.
Developers who sign up to offer Search Ads are being given a free $100 credit to get started with the program.