I have an iMac running High Sierra. The Library folder under my user account is listed as 90 GB. I was looking for the major factors in this and the folder Mobile Documents is 50 GB. When I click on the folder it opens the iCloud drive on my machine! They are apparently the same. I recently read the icloud drive was like Dropbox or Google drive. You can store things there to save disk space. I upped my icloud storage but now it looks like it uses space on my internal drive anyway. What gives?
It depends on your settings. If you have it configured to store everything locally, you won't save any space on your internal drive by moving files to iCloud Drive. If you have it set to store them only locally as needed, then you can save storage space that way.
You can change the settings in System Preferences > iCloud and then "Options..." for iCloud Drive. At the bottom you'll see "Optimise Mac Storage" that you can check to request that files can be stored in the cloud only.