The adjective fecund describes things that are highly fertile and that easily produce offspring or fruit. Rabbits are often considered to be fecund animals, and you may hear jokes in poor taste about people reproducing like rabbits if they have a lot of children.
The word fecund comes from the Latin word fecundus, meaning fruitful. But the English word does not just describe something or someone fertile, the adjective fecund can also be used to describe someone who is innovative or highly intellectually productive. Your fecund imagination will be an asset if you have to tell ghost stories around the fire at camp while eating s'mores but that same fecund imagination could be less helpful if you're at home alone on a stormy night and you think you hear a knock at the door!