Natural resources are things like land, gas reserves that are still in the ground, fish in the seas and virgin forests.
Not all naturally occurring goods are considered to be Natural Resources. For National Accounts, to be a Natural Resource, it must have come into existence without any human production. If a forest has been cultivated, or cattle have been bred, then that is a Produced Asset not a Natural Resource.
A Natural Resource also has to be something that can be owned as an asset: the open seas and the air cannot be controlled and so cannot be owned. However the right to fish in the open seas can be controlled. Rights to fish are Permits, which are a different kind of asset to Natural Resources.
In Ireland, groundwater used by houses with their own wells is not a Natural Resource because groundwater is not so scarce that people have claimed ownership rights. In other countries, water may be scarcer and the right to use groundwater may be owned and traded. In general, Natural Resources are only things that are owned by someone: if they are not owned then they are not in this category.
They are also economic assets: they have value that can be sold or produce goods and services that can be sold. There are many natural good things we enjoy which we do not put a price on, but National Accounts only describes economic value (it does not try to describe other aspects of our welfare).
The income paid to the owner of Natural Resources by the user is called rent in National Accounts, which is part of Property Income. The rent on Natural Resources is treated differently to payments for using other resources like hiring a car, leasing an aircraft or renting an apartment, which are all treated as the owner producing a service.
Natural Resources are a kind of Non-Produced Non-Financial Asset.