Asset bundles are Unity Pro features only. Two main ideas of the asset bundles are:
Easily download content in your application
Uploading new content in your application
Unity allows exporting your assets as files, which are known as asset bundles. Your application can download these compressed files whenever needed. This approach will reduce your final build size by streaming in: prefabs, animations, binary files, textures, audio clips, meshes, and scenes, where asset bundles will be utilized. All other asset types are supported by Unity. For binary files, you should set the extension to .bytes and Unity will recognize these files as TextAsset. To use asset bundles, you just need to create them and upload them to your server. In the Unity Editor, you can build asset bundles from your assets in the scene. In a situation when you need to upload your asset bundles to the server, you can use any data communication protocol; for example, SSH, FTP, FTPS...