Although Apple first debuted iCloud Drive in iOS 8 last year, it was implemented merely as a hidden file system designed to be used only within apps specifically designed to support it; unlike on OS X Mavericks, there was no iOS-based option to directly browse through your iCloud drive, making it a completely impractical replacement for something like Dropbox or Google Drive. The good news is that Apple has opened this up with iOS 9 by adding a standalone, built-in iCloud Drive app, providing direct access to everything you have stored in iCloud Drive, along with some basic file organization and sharing features.
For whatever reason, Apple has chosen to keep this new app hidden by default, leaving users with the same sort of experience they had with iOS 8. However, if you want to get direct access to what’s in your iCloud Drive, a new “Show on Home Screen” option can be found under Settings, iCloud, iCloud Drive to enable it. The icon will appear in the next available space on your home screen, and from there can be accessed and moved around like any other built-in app.