Essentially you add files and folders to Dropbox as normal, and after they are uploaded you use the Selective Sync feature to no longer sync that folder with your Mac. Thus all that data is just in the Dropbox cloud.
The problem? This works well for iOS, since you can browse all the files in the Dropbox app, but it doesn’t work well on the desktop where you would have to browse those files on the Dropbox website. So yes, the files are not on your Mac (or any Mac), but they also are now not easily useable on any Mac.