@Domitilla wrote:
... if I buy 2 Tera of space on dropbox but my tablet's hard drive has only 500 GB, what happens?
If the tablet is running a desktop operating system, such as Windows, you can install Dropbox and make use of the ability to mark files and folders as Online-only. Only the files that you open would sync down to the local drive.
If you have a mobile operating system such as iOS, Android, or Windows S Mode, then the Dropbox app doesn't store files locally, unless you mark them as available offline. The apps simply let you access the files in your account, downloading (temporarily) only those files that you open.