OBB expansion files (Opaque Binary Blob) are extra files added to the APK, which are used in the Android application installation process and have an .obb extension. While the APK files store an app’s basic components, OBB files save all additional information and all necessary data for correct functioning. They were introduced onto Android in the operating system’s Gingerbread version 2.3.
Users are normally unaware of OBB files that are downloaded onto the device once the app has been installed. However, you might find some applications that are distributed with the OBB files already included, these are called XAPK files, and it is even possible to download these OBB files separately.
OBB data can be found in cache data - maps explored, levels achieved, enemies defeated, special abilities… It is not an app as such, but rather extra information related to an app.
An app’s OBB files are accessible on an Android device using a file browser, so you can work with them, copy them and paste them wherever and whenever they are needed. In system folders you will find them for every app that you have installed. The OBB data is in a folder with that name. But do be very careful when manipulating this kind of file, because you could corrupt your app and game installation and cause them to not open correctly if you are not careful. First learn about how to manipulate this kind of app data.