You can check the boxes next to the content that you want to back up to the cloud and once enabled, depending on the space you have on the iCloud, the files on your system will start getting back up on your iCloud account.
Now, please be advised that at the start, you have 5GB of free iCloud storage, but this quickly burns up because everything gets backed up on the cloud when you bott up the Mac. So, if you are fully committed to using iCloud, you can expand the storage by paying a monthly fee.
