Natural capital is a metaphor for the mineral, plant, and animal formations of the Earth's biosphere when viewed as a means of production of oxygen, water filter, erosion preventer, or provider of other ecosystem services.
But the benefits which humans have from nature are many. 17 of them were closely looked at by Robert Costanza. These benefits are in some ways similar to those that owners of "capital" have as their capital produces more goods, e.g. a factory which produces automobiles just as an apple tree produces apples.
This is an approach to ecosystem valuation, an alternative to the traditional view of all non-human life as passive natural resources. But human knowledge and understanding of the natural environment is never complete, and therefore we cannot yet know what natural capital means exactly.