What the lines represent. In the diagram below, left of the vertical line represents above average people, to the right of the vertical line are those who are below average. The line at the bottom is zero people.
Most people are near the center, where the curve is highest.
Few people are at the extremes, where the curve is lowest. What is being measured. The curve could, conceivably, also measure socially beneficial behavior, with individuals who are absolute saints on the extreme left and psychopathic killers on the extreme right. Comparing races or nationalities. People who have engaged in the strange business of measuring the intelligence of races or nationalities have come up with results of a percentage point or two or more difference between the groups.