Canine Hip Dysplasia (CHD) is one or more genetic defects in a dog's hip(s). It can be that the hip socket is too shallow or that the ball is too small or shaped oddly (or missing altogether) or some or all of the above.

This is why that breeders (of purebred) dogs normally have the hips OFA certified on their breeder dogs. It markedly reduces the chances of any of the pups being dysplastic.