This is a high-quality crushed rock ballast used by model railroaders and diorama builders to realistically model track beds and gravel roads. It is colorfast, lightweight, and typically applied using a scenic cement.
The "Fine" grade ballast is explicitly optimized for N scale layouts, but it can also be mixed into HO scale projects for variety in texture.
For the cleanest look, owners recommend pre-wetting the layout with "wet water" (water mixed with a drop of dish soap) before securing the ballast with an aggregate adhesive like Woodland Scenics Scenic Cement.