Today, these are commonly thick, dry, oatmeal biscuits.
Jane Grigson's 'English Food' gives a report of them being made from yeast-raised oatmeal batter thrown onto a heated griddle in a narrow strip, so that it immediately puffs up with steam, "when baked it is damp and flexible, and is hung on the wooden clothes rail before the fire to dry, or on lines across the kitchen ceiling. It must be crisped quickly immediately before it is to be eaten." (Grigson 1974)