DOSBox, in an effort to emulate a class MS-DOS environment needs some location to emulate a virtual Hard Drive. Commonly MS-DOS users would be greeted with a
prompt once a computer is done booting. The hard drives currently in your system are most likely many times larger than the kinds of hard drives that existed back when MS-DOS was commercially available. Also, they contain software that is not suited for DOSBox. In order to create a realistic (and safe) environment to run your DOS software you should define a path somewhere on your hard disk that will be treated as a virtual hard disk.
Ideal locations for a virtual hard disk are
Note that you can have more than one Drive defined if you want to recreate complex configurations, but it is generally easier to have a single drive that will be mounted a your C: drive.