Sometimes you can look at two slices of pie and tell which is bigger. Sometimes you cannot.
Home Experiment: Slice a pie up into what you think are equal sized pieces and line them up according to size. Many look the same. If we want to arrange the pieces according to size, we need another way to tell how big each piece is. A weight scale will do quite well. Now - lets look at what we would find if we weighed each piece.
There are big and little pieces, but you can see that the number of pieces in each step of the graph (weight group) varies from the largest piece to the smallest piece in a fairly regular and symmetrical pattern. This is the Distribution of the weights. The curve is what we would expect if the Distribution was a ‘Normal’ distribution.