Dropbox's new feature gives you all your stuff without filling your hard drive
Basically it's a way to have access to all the files you need from Dropbox whenever you need them without having to also use up all of your hard drive storage.
It works by showing your files in a local folder as normal, letting you browse with ease, but they're not actually located on your computer. Instead the documents, images, spreadsheets or whatever are ready and waiting in the cloud.