What are OBB files, what do I do with them?

OBB stands for Opaque Binary Blob. Google Play mandates that APK files may not exceed 50 MB in size. If an app is larger than that, then it must use expansion files.

To install an OBB on your device, use a filemanager to copy it to:

[shared-storage]/Android/obb/[package-name]/

[shared-storage] is the path to the shared storage space. [package-name] is your application's app id.