This API is used to manipulate assets in a storage - list them, update, delete, organize in folders, etc.
You may think of it as an abstraction of a file system. The assets (such as designs, fonts, images, etc) are similar to files, they can be organized in folders, you can perform the CRUD operations on them.
Note, on practice you won't want to create files using this API. The reason is that you are usually need not just to put a file to the system, but also get its metadata and a preview image. So to upload anything to the storage, we recommend using the import operation of the AssetProcessor API service.