Hi "Metro"
As no-one else has posted an answer to your question yet - thought I'd have a go at answering. I am not an expert in this by any means - there are many other members of theboard who do know much more, but a very basic explanation, as I understand it is:-
Certain breeds are prone to a problem called Hip and/or Elbow Dysplasia, where the sockets and balls are not properly formed to make a nice fit and so causes walking problems, arthritis and the such like in later life - in some cases it is very severe and the dog can be crippled. Hip Scoring uses an x-ray which is then scored, ie marked by an expert looking at the formed ball and socket to see how well they have developed and fit etc. the lower the pionts scored, the better the joint. Each breed has it's own acceptable score. Any dog or bitch that scores highly should not be used for breeding as they are likely to pass on the fault.
The scoring cannot be done until the dog/bitch is of a certain age and that depends on the breed for maturity etc. Also I gather that a bitches seasons can affect the accuracy of the x-ray, so timing is critical as once you have a score you are stuck with it.
All helps improve the quality and therefore health of the pups that are bred. Unscrupulous/non-reputable breeders often don't do it, as long as they have buyers, they are not bothered. Having said that, many reputable breeders don't do it as they know the quality of their stock.
hth and that it is correct. I'm sure others will correct where necessary.