The app needs to be unhidden manually from the iCloud Settings menu. It lets you browse through the files you have stored in iCloud Drive, open files, delete files, move files, and share files via the iOS share sheet.
This app makes iCloud Drive a more viable replacement to Dropbox, letting users easily access their files on the go.
Take a look at some screenshots below and please follow iClarified on Twitter, Facebook, Google+, or RSS for more updates on iOS 9. You may also want to check out the official iOS 9 announcement here.