here is a quick explainer. Before app bundles developers either created a single "fat" APK with all the libraries and resources in them or manually managed multiple APK variants (for example arm64 320dpi x86 320dpi arm64 640dpi etc.).

The new App Bundles allow developers to transfer the burden of dealing with variants to Google which then splits the app release into multiple chunks - hence the term split APKs. Each release then consists of a base APK and one or more APK splits.

For example a single release may now arrive as 5 files: base.apk + arm64.split.apk + 320dpi.split.apk + en-us.lang.split.apk + es-es.lang.split.apk.

Unfortunately you can't install all these APK splits by just tapping them on your device - you can only install the base APK which will then crash due to missing resources