Sour mashing is a process used in the distilling industry (predominantly American Whiskey) that uses the leftover material from an older batch of mash to start the fermentation of a new batch - much like the starter in sourdough bread. The spent mash, (also called Backset or Stillage) helps lower the PH in new batches, regulate bacterial growth and ensure flavour continuity from batch to batch.
Some distillers use up to a third of Backset in their new ferments (although it’s more commonly a quarter). While this process gives added character to the resulting whiskey – critically, it doesn’t mean that once distilled and matured the whiskey inside is sour.
See Backset, Dunder for more related to the process.