Fruitfulness is a state of being extremely productive or prolific. Your five-year-old cousin's fruitfulness as an artist means that she goes through a box of crayons every single week.
An apple orchard's fruitfulness literally results in a lot of fruit — the more fruitful it is, the more apples it produces. There are other, more figurative types of fruitfulness too. If your aunt and uncle have a huge family, with 12 kids, people might remark on their fruitfulness. And if a poet goes through a period of creating dozens and dozens of poems, that's a kind of creative fruitfulness.