When trying to describe the quantity or quality of something, whether a chemical substance or physical
phenomenon, it is helpful to have some standard measure to refer to. A unit is nothing more than a standard by which a measured value can be described. For instance, in the old English system, a foot was just that--the length of a man's foot. This was helpful in that when a distance between, say, a house and a water well was described in feet, one who had never seen the dwelling in question would have an instant idea of how far away the well was, based solely on a verbal description. The concept today hasn't changed, although our standards make better scientific sense and are more exact.