Tree
Heartwood and sapwood
Sapwood is the living outer wood of a trunk that carries water, and heartwood is the dead inner core that gives the tree strength.
A trunk has two main wood zones. Northern Woodlands says sapwood is the living, outermost portion of a woody stem or branch, while heartwood is the dead, inner wood, which often comprises the majority of a stem's cross-section.
Sapwood carries water from the roots to the crown. As a tree ages, inner wood stops moving water and becomes heartwood. Heartwood is often darker, but color alone can mislead. Not all heartwood is dark.
On a stump, the rings you see are these two zones. A stump grinder cuts through both. Heartwood can be dense or rot-resistant in some species, and decay fungi can attack either part. See stump fungi and hardwood and softwood.