The phrase "heel bone" refers to the bone located at the back of your foot, specifically beneath your ankle. It is responsible for supporting your body weight and helps you walk and run. Full definition
«It puts the final nail in the anti-dinosaur coffin» says Jacques Gauthier, a vertebrate paleontologist and professor at Yale University «The dinosaurian ascending process is retained in all birds, though it has changed its association from ankle to heel bones in neognath birds.» (sciencedaily.com)
Li (the guy that got launched into the air) was diagnosed with broken heel bones and fractured vertebrae, a miracle considering the size of the impact. (wtf1.com)
Some pointed out that the ascending process in most birds was a projection of the neighbouring heel bone, rather than the anklebone. (sciencedaily.com)