How to move files from my laptop to dropbox to free up storage on laptop?

I found some useful answers below: could some explain how to use Selective Sync? Thanks
If you want to save space on your local drive then you can use Selective Sync to remove the local copy of certain folders. Just know that you'll only be able to access those folders through the Dropbox website if you remove them from your computer. Also, make sure that a folder is completely synced (green check mark) before you remove it with Selective Sync.