When we speak of the weight of a letter, we are speaking, of course, of the thickness of the lines that compose it. The letter is considered light when the inked area is less than that of the paper surrounding it. The opposite is true for heavy weights in which the inked areas fill up most of the space.
Light and heavy are contrasts as effective as night and day, a device used by artists to dramatize their subject. Contrast of weight provides direction signs for the reader when looking for specific information such as headings or keywords etc.