Drawing value is understanding the balance between light and dark in our art work.
If you can find the light on the opposite side is the dark.
This is why it is important to find the source or create a source from where the light comes from.
Without that light source or a light source you would not be able to see any objects in your picture as the light helps define what you are looking at.
VALUE IS MORE IMPORTANT THAN COLOR!!!
Yes, but how do you draw light and dark? It's like driving a car. The more pressure you apply to a gas peddle in a car the more intense your speed will be.
Applying pressure to the pencil will make your line and shading work more intense.