Originally Posted by The Fifth Horseman

In order to understand what mounting is, you need to realise DOSBox does not have access to your entire hard drive, nor does it need to have access to entire partitions either. Mounting a directory - or a drive - makes it accessible to DOSBox as if it was an actual partition or drive in the emulated machine.
TL;DR : It tells DOSBox what it can access on your hard drive.

As to how to do it, you should be able to configure your mounts in the game's D-Fend profile. Can't say for sure beyond that, since I don't use D-Fend. The actual command to mount a directory is described in detail on the DOSBox wiki or in the readme that came with the program.