Application Resource Bundle (abbr. ARB) is a localization resource format that is simple (based on JSON), extensible (vocabulary can be added without affecting existing tools and usage), and directly usable (applications can access the resource directly from this format without converting to another form).

In ARB, localizable resources are encoded as a JSON object. Each resource will have a resource entry identified by resource key, and an optional resource attribute entry with resource attribute key.

ARB files are used to localize apps built with Google's Mobile App SDK called Flutter.

Documentation

Official ARB Documentation
Official Flutter Localization Documentation