The feature enables Google to deliver in-app content in search results on mobile devices (specifically Android devices for now). For example, if you search for "Dee Barnes," you might get a result from Wikipedia. With app indexing, Google will give you the option to open the app from the result as opposed to going to a mobile web version.
The feature requires app developers to be on board, so Google has announced specific publishers with content in different languages that are now taking advantage of app indexing. These include: Fairfax Domain, MercadoLibre, Letras.Mus.br, Vagalume, Idealo, L'Equipe, Player.fm, Upcoming, Au Feminin, Marmiton, and chip.de.
Google has also translated its developer guidelines into eight more languages (Chinese - traditional, French, German, Italian, Japanese, Brazilian Portuguese, Russian, and Spanish), so that should help too.
Google has a form here where you can request to participate in App Indexing. The company notes that it has added a few new apps in the U.S., including Walmart, Tapatalk, and Fancy.
Google promises a session for developers at Google I/O dedicated to "the future of apps and search".
Internet giants like Google and Facebook are working to make mobile apps more web-like. At Facebook's recent developer conference, the company announced App Links, which enable apps to link to content within other apps.
Image via Google