What is main.obb and patch.obb? Why does they generate only when I package Android app using "Use APK Expansion Files"?

Google Play currently requires that your APK file be no more than 100MB. To make this process easier for you and more pleasant for users, Google Play allows you to attach two large expansion files that supplement your APK.

When you use expansion files, one file is the main file and the other is an optional patch file. Optional patch files are usually used for small updates to the main file.

The main expansion file is the primary expansion file for additional resources required by your application.
The patch expansion file is optional and intended for small updates to the main expansion file.

When you are publishing your app on Google Play Store, you will need to upload your main.obb file with your .apk file.

Normally main.obb file contains the all media files hence you will need to upload main.obb file too when you publish to the play store.

Please follow this instruction to upload obb file that contains all media files. 
https://support.google.com/googleplay/android-developer/answer/2481797?hl=en
https://developer.android.com/google/play/expansion-files.html