Bones, usually pork, chopped into pieces with some meat and gristle still on, sold by butchers for soup. They are blanched, then simmered for hours into a rich broth, and the meat left on them is eaten too.
肉骨头 emphasizes the bone-with-meat aspect - distinct from plain 骨头 (bones, possibly bare). Used in cooking contexts (broth-making) and casually for the kind of chewy bone scraps fed to dogs.