Heterosis, or "hybrid vigor" is an increase in "quality" of a certain amplified gene/trait/characteristic or any other biological quality in the hybrid offspring.
G.H Shull described how the "physiological vigor" associated with the organism's rapid growth, is positively correlated with the ranging degree of dissimilarities found in the biological parents of the offspring. Shull first proposed the idea that the further away biological parents were genetically from each other, the more biological "vigor" could be inherited from either side. Having two pools of different genetic material to pick and choose from when your body is constructing life would certainly increase the chances of your offspring being biologically competent.
Heterosis & You. Edit
Heterosis can be closely studied when looking at the Major histocompatibility complexes and how they pass down to offspring. Each parent donates both MHC1 and MHC2 class molecules to their offspring. Breeding between genetically diverse and different individuals would decrease the chance of passing the same or similar allele; allowing a more diverse range of peptides to be presented on the immune cells ultimately leading to a greater response.
Heterosis everywhere else. Edit
Heterosis is exhibited everywhere, ranging from cattle and corn to humans and hens. Linking Iowa Stiff Stalk Synthetic maize & Non-stiff stalk to form a perfect hybrid, to crossbreeding Black Angus and Hereford to produce a hybrid known as "Black Baldy". Even within poultry, heterosis has been used to create sex-linked genes where they can create hybrids to sort off color at one day old.