Apple's new desktop software, MacOS Sierra, takes the two folders you likely use the most -- Desktop and Documents -- and syncs them with iCloud Drive. That means any files you have stored in either spot are accessible from your other iCloud Drive-enabled devices, including your iPhone, iPad, other Macs and even Windows PCs.
After you install MacOS Sierra, one of the setup screens will ask you if you want all your files in iCloud. That might be overstating it slightly, because the fine print states that all your files on your Desktop and in your Documents folder will be automatically uploaded to to iCloud Drive.
Setting up iCloud Drive sync
If you missed the iCloud setup screen, don't worry -- you can enable these two folders to sync via iCloud Drive after you've installed MacOS Sierra.
To sync the Desktop and Document folders, open System Preferences and click iCloud. Next, click the Options button for iCloud Drive and make sure the box is checked for Desktop & Documents Folders.
Using iCloud Drive sync
With your Desktop and Documents folders synced via iCloud Drive, you can access files in these folders from iOS devices using the iCloud Drive app. Just look for the new Desktop and Documents folders in the app.
On other Macs, open Finder and click iCloud Drive on the left panel. You'll see the Desktop and Documents folders in iCloud Drive alongside any other folders files you have stored there.
On PCs, you can access your synced files from iCloud.com or the iCloud for Windows app.