Value (how dark or light) and contrast (difference between two values)

Positive space (what your object is) and negative space (everything else)

Shading of 5 Basic Geometric Shapes, with light source in top left

1. Cube - flat faces
2. Sphere - circle when unshaded. shadows and highlights give it 3D illusion. roundd shadows.
3. Cylinder - part flat, part rounded like sphere
4. Cone - like a cylinder, but tapers to the top point
5. Pyramid - none of the middle value. just lighter and darker.

https://www.piskelapp.com/p/agxzfnBpc2tlbC1hcHByEwsSBlBpc2tlbBiAgOCwhbe5Cww/view