From the University of Nevada, Las Vegas Food & Beverage course notes on Rum -
Dunder is a residue from previous distillations and is allowed to undergo a bacterial fermentation. In a technique similar to that used to make "sour mash" bourbon, the dunder is used to begin new alcoholic fermentations. Like sour mash it helps control the pH and buffers the fermentation as well as adding in flavor development.
Dunder. The residue from a previous distillation which is introduced during fermentation. The dunder undergoes a bacterial fermentation prior to use. Its value is in the provision of bacteria that aid fermentation and create valuable flavor and odor compounds. Dunder contains up to 3% organic acids, mostly acetic and butyric, and aids in reducing the pH (increasing the acidity) of the mash before fermentation.