Many of the finest artists of contemporary times consider one aspect of art to be far and above the most important part of a strong piece. Without an understanding of value, we cannot create art that shows form or depth to a satisfactory degree, and our work will appear flat and dull. It's a huge subject, and an even more difficult one to master (something I will no doubt spend a lifetime trying to perfect myself) but a simple one to get a grasp of.
Put simply, value is the term we use to describe an objects relationship to light. Known also as 'tone', value communicates to us where an object sits on a black/grey/white scale. It shows us the form (3-Dimensional shape), depth and distance of an object. If line drawing is 2D, value is what we use to make our paintings come alive in three dimensions!