iCloud Drive is a Dropbox and Google Drive-type service from Apple that lets you store documents in the cloud, making it super easy to synchronize them between your iOS and OS X devices.
You can access it on your Mac with an icon that shows all the documents you’ve stored in iCloud in a folder-like structure.
Prior to iOS 9, in order to access these documents on your iPhone or iPad, you’d need to open an app that supported iCloud Drive on your device.
Not anymore. iOS 9 comes with its own iCloud Drive app, and here’s how to enable it.