Sapwood is the outer part of the tree which has not become heartwood. As the name implies, it carries sap up and down the trunk.
If you have used hard maple for a neck, you have used sapwood. In maple, the heartwood is less stable, and darker colored, so the good stuff is selected for sapwood. In woods like walnut and cherry, it is selected for heartwood, for the dark color. In these woods, the sapwood is only an inch or two thick.
It is confusing, because the term "heart" also refers to the very center of the tree which is usually very unstable and prone to cracking. In higher grade lumber, this is left out entirely. The "heartwood", between the "heart" and the sapwood is stable in most woods, maple being an exception.