Cryptorchidism in simple terms means having a retained testicle(s). It means the testicle hasn't moved into its proper position in the bag of skin hanging outside the body and has stayed inside the body. This condition, while not overly common for puppies, does occur occasionally in male puppies. At the 6 week vet exam, the vet will check to see if both testicles have descended. It is more common with this condition for one to drop and one to be retained but in about 10% of the cases, both can be retained. The testicles may still drop until around 6 months of age. So, if they haven't by the 6-week exam, they still may in the coming months. It is less likely, but they may. This is not actually a health concern, just something you will need to be aware of. The reason is because if one or both did not drop into place, then they will need to be removed at the time of neutering or sometimes the surgeon will maneuver it into place and stitch it to hold it in place. Undescended testicles are highly likely to become cancerous down the road so they must be removed. The testes are meant to be outside the body so they can be cooler just like with humans. Therefore, if they are still inside, it causes their temperature to stay elevated, thus, potentially causing them to become cancerous. Once the undescended testicle(s) is removed during neutering, there will be no further health concerns. 

I will always let everyone choosing a male puppy know if any of them do not have both testicles descended at the time of choosing so you are aware. It's not necessarily a reason to not choose that puppy, you just need to know because the cost of neutering will be a little higher with also adding in removal the testicle(s) should it still not drop in the 1st few months.

The only reason you should not choose a male with undescended testicle(s) is if you were purchasing him for breeding purposes, as a cryptorchid puppy cannot be used for breeding. This condition is genetic, but both sire and dam have to carry the gene for it to occur in the puppies. This gene cannot be tested for so it is not known if the adults may pass it on until a litter of puppies have one or more males with it. Therefore, it can show up unknowingly in any litter. 

I have occasionally had 1 or 2 males in a litter have cryptorchidism over the years but not very often and I actually haven't had any in a few years now!

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https://vcahospitals.com/know-your-pet/retained-testicle-cryptorchidism-in-dogs

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