The .OBB extension refers to files of the type, Android Opaque Binary Blog files. These files are mostly used by the in-app features which belong to the apps downloaded from the Android Play store.

This blob file has the ability to store data in binary format and can store some additional information that is not stored in the app’s storage. The application developers can use this blob file type to attach the additional data that cannot be stored in the limited storage that android market offers. Most of the times, sounds, mission files and other interactive data is provided in these blob files.