Ok job done. Whatever a drop box is
It's basically online storage, that is automatically synced to the dropbox folder on any computer(s) that you install the program on and login to.
I use it mostly for projects I'm currently working on (that don't involve stupidly large files!), as it means everything gets automatically synced between my desktop and laptop, and if need be I can access it via the web, or my phone. Saves the usual hassle of remembering to copy stuff you're working on between computers.
Plus it means you've always got a backup should things go tits up.