Also known as heterosis, the term hybrid vigor in dogs describes improvements in form, health or function as a result of crossbreeding. You may see this used when some breeders describe their Doodle puppies. Because many people believe that designer dogs are inherently healthier than purebred dogs because they do not have the same predispositions for inherited conditions, breeders will use this as a marketing trick- and most don't know they are using it in the wrong context. But are mixed breeds or designer dogs healthier than purebreds? A few decades ago- maybe.
Many purebred breeders utilized linebreeding. Linebreeding was when father/daughter or mother/son pairings were commonly done to lock in desirable traits. But it also locked in undesirable traits and created health conditions and diseases. Puppies ended up inheriting undesirable genes that affected their health because parents were closely related and carried the same mutated genes. It also created a high COI (coefficient of inbreeding).
When purebred breeders noticed health issues arising they would outcross. They would bring in another breed to increase the genetic diversity of their breed to reduce the health issues that were there. This is where the term hybrid vigor came in.
We now have genetic DNA testing that shows each dogs COI as well as any health conditions or diseases the dog may be carrying. We don't breed dogs together that have undesirable traits - purebred or otherwise - because we can now test them before we breed them and can now avoid many of these health conditions and diseases. Decades ago we didn't know this. We now know better- so we do better.
Hybrid vigor does exist but just because a dog is a mixed breed does not automatically mean it is healthy. It is likely healthier than a linebred dog but that doesn't equate to a healthy dog like they want you to believe. If the breeding dogs have not been fully health tested prior to breeding, them your mixed breed puppy can still have all kinds of health issues. Genetic and OFA health testing still need to be done prior to breeding to ensure health puppies- purebred or mixed.
So please don't fall for the "our dogs are healthier because they have hybrid vigor". Ask questions and do your research. Make sure the breeder health tests their dogs!
For more reading check out this journal by UC Davis on Hybrid Vigor!