The word fertile means "able to reproduce," but like so many words, that's just the beginning. The fact that she gave birth to eight kids was proof that she was fertile; her fertile imagination explained their unusual names.
The distance between fertile's literal meaning (able to make babies) and its figurative ones (productive, prolific, full of potential) is small. A child's mind is a fertile place; an idea can grow there very easily. Rabbits are famously fertile creatures; they can spawn several generations in a matter of months. And "fertile ground" can be arable land or a situation that provides the perfect opportunity. The Latin root, fertilis, means "bearing in abundance, fruitful, or productive," from ferre, "to bear."