Dropbox is an app that syncs files inside a specific folder between computers (and phones), which means you would have access to the same set of files on more than one computer and they'd be updated automatically on all devices each time the files are modified on one device. You need to make an account and also be able to install Dropbox on the devices you want synced up, though.
If you own more than one computer that you play games on, you can install Dropbox on them and keep your save data inside the folder that Dropbox syncs, so you wouldn't need to manually copy over the files each time you switch computers because Dropbox would update the files for you.
It's useful for games that don't have cloud-saving features.