Value is lightness or darkness. In a simplified drawing with one light, the lit side can be lighter and the side away darker. Overlap is another depth clue: an object in front hides part of one behind. These are drawing models, not rules that every artwork must follow.
Worked example
With a lamp at the upper left of a matte sphere, place a light area toward that lamp and a darker area toward the lower right. A cast shadow falls on the surface away from the light. Real reflections can make the pattern more complex.
A Pencil Still Life
On paper, draw a box and a ball that partly hides the box. Mark a light source at the upper left. Make three labeled value patches: light, middle and dark. Shade the ball using those values and add a cast shadow on the surface away from the light. Explain one overlap and one value choice. No upload is needed; assess these observable choices, not a preferred art style.