What is the meaning of Draff?

A byproduct from a brewery, a distillery, or both, often fed to pigs or cattle as part of their ration; often synonymous with distillers' grains, brewers' spent grains, or both (when not differentiated); usually differentiated from potale, at least in technical use, although broad, nontechnical use has often lumped all such byproducts together, especially in the past.

Source: wiktionary.org