The AAB (Android App Bundle) is a publishing format for Android applications. It's not directly installable on a device like an APK. Instead, it's a container that includes all the compiled code, resources, and native libraries of your app, but it defers APK generation and signing to Google Play. When a user downloads your app from the Play Store, Google Play uses the app bundle to generate and serve optimized APKs for each user's device configuration. This process is called Dynamic Delivery. This means users only download the code and resources they need to run the app on their specific device, reducing the app's download size and installation footprint. This leads to faster downloads, saves storage space on users' devices, and reduces data usage. App Bundles support features like dynamic feature modules, which allow you to deliver certain features of your app on demand, further optimizing the user experience. The AAB format is designed to replace the traditional APK publishing method, offering significant advantages in terms of app size optimization and delivery efficiency. It is the recommended publishing format for new apps on Google Play.