Cow bones, mostly marrow-rich leg bones and neck bones, sold cut into pieces. They are blanched, then simmered for hours into a rich broth, or braised or grilled and the marrow eaten.
The base of many Asian noodle soups. Lanzhou 兰州拉面 simmers beef bones for hours; Vietnamese phở and Korean 설렁탕 likewise. The marrow (骨髓) is prized separately. In casual speech you'll often hear 牛骨头 (with 头 suffix).