Compare to Selective Sync (requested at the link below) Smart Sync (Dropbox naming) allow to see all the folders and files even if they're not sync locally (so you can search, have a preview…).
And it allows in a right click to quickly bring back the file in local or to put it online.
It will be great to have a similar feature!
Here the Selective Sync feature request:
Here the Dropbox Smart Sync explanation page: