Hip Dysplasia is an inherited condition that means the thighbone doesn’t fit snuggly in the hip joint.
The symptoms are generally pain and lameness which will vary in severity and can be present in one or both rear legs.
Sadly, it’s not uncommon for the disorder to lead to hip replacement.
Make sure you ask any breeder if the puppy’s parents have been tested and cleared for Hip Dysplasia. If they have not, or the issue is present in one or both, find another breeder.