Value is one of the most important things to consider when creating a painting. Many artists will say that value IS the most important aspect to focus on when observing a scene.
Value is, simply put, how dark or how light the tone of a specific object or section of an object is. Every scene has it's darkest part and lightest part. In between you will find a variation of tones. All of these are different values.
The illustration below shows a value scale of the color yellow ochre.
The value scale starts with pure white and ends with black. The same color was shown as a grey value scale. Every scene should be viewed as if it was a black and white photo. This way you will immediately point towards the darkest and lightest sections of the scene.
When painting or drawing you need to accurately place these values. Once you have succeeded in this, your choice of color needs to be determined. But focusing on the value is the first important thing to do.
Above you will see a painting done in a monochromatic manner. That means a limited palette was used for the painting. In this case a variation of the color yellow ochre. The painting shows how different values were placed by the artist. If you move your mouse over the image you will see where each part of the value scale can be found.
A way to practice seeing value correctly would be to create a few paintings by using a monochromatic palette. Choose any color. Use black paint to darken the color and white paint to lighten it. Your pure color will fall in the middle of the value scale.
Many oil painters still uses the old masters technique where the artist would first paint the dead layer. The artist would paint the entire painting with black and white paint. Gray paint would be used to show the mid tone values. This is called the dead layer. Then color would be glazed over this layer.
I illustrate this technique in a tutorial you can find here: How to paint like the old masters.