Dunder is the liquid left in a boiler after distilling a batch of rum. Similar to backset in American Whisky (used for Sour Mashing), the stillage from a rum distillation is commonly poured back into a new wash to add flavour characteristics.
Not just helpful in lowering the PH and helping to create optimal fermentation conditions, Dunder contains by-products that help in future fermentations in other ways, including dead yeast cells, which act as a yeast nutrient. While less common practice today, historically after a run the Dunder was sent to a "dunder pit” left to settle and develop bacteria that would aid in concentrating certain esters (like Butyric acid) – key to the flavour of some rum styles.
Typically, especially for the likes of Jamaican rum, around 20% of a new wash is dunder from one or more previous washes.