When speaking about color terms, value refers to how dark or light a color is. This is easiest to see in grayscale or when you have just one color, as in the example below.
Full color images have a value range too, but it can be more difficult to see those values when you have lots of colors. Squinting your eyes can help you see the values a bit easier.
Here, I've changed my color photo to black and white. The values are the same in both versions of the photo.
Value is important because it helps us decipher an object or scene. We can determine an object's shape, using value. For example, the tomatoes in this photo look round because of the change in value from light to dark. This shift in values is known as a form shadow.
Value also helps our brain determine how objects relate to one another in space.
A change in value is a way to decipher depth and perceive distance.
As an object moves back in space the value becomes lighter. If you compare the mountains in the foreground to the ones in the background, the background mountains get lighter and lighter the further away they are.
In some ways value is more important than color. If you get the values right, you can put almost any color into a scene and it will make sense. Here I've changed the colors of my original image, but the values remain the same.
Value is important because as artists we can't create the illusion of depth or three-dimensional form without it. Understanding value, and getting the correct value relationships in a scene is an essential part of creating successful paintings.