The spent stems of wheat or oats once the grains have been threshed, harvested by hand or machine,, which are then cut and made into bales for packaging and for stable floors.
In earlier times straw was the main stuffing for mattresses and sometimes upholstery as a less expensive and / or more easily available substitute for horsehair, wool, or down.
Floor mats in rural communities especially were woven of straw and changed every year, as they still are in much of the world. Straw paillasse (mattresses) in Europe are for many only a generation or two away, and still used now for temporary bedding in the lambing shed, for example.
I slept on a feather mattress over straw bales a couple of years ago in a smart but pretty unusual hotel, and it was a much more pleasant experience than I had expected–in fact, very comfortable and with lovely smell reminiscent of summers spent on the farm.