As it applies to goldendoodles, hybrid vigor basically means that the mix-breed offspring of two purebred dogs has a stronger genetic make-up than either pure-bred parent due to genetic diversity; that is, the stronger, healthier genes are passed down and tend to be dominant in mixed-breed dogs, such as the goldendoodle.
For example, common health risks in pure-bred golden retrievers include allergies (“hot spots”), thyroid problems, heart and/or eye problems, and hip and/or elbow displasia. Poodles are at risk for these same genetic issues as well as epilepsy and adrenal disorders. Cross-breeding pure-bred dogs promotes genetic diversity because it allows for whichever parent gene is stronger and healthier to be passed down.
This is another reason an F2b dog is superior to an F1 cross. With an F1, there is always an off-chance that two seemingly-healthy pure-bred parents are both carriers of unhealthy genes that are not expressed, meaning each parent could pass down the non-dominant/non-expressed unhealthy gene to the offspring, who would then inevitably have related health problems. F1b or F2b dogs have less chance of this happening.