When the supply curve and demand curve are overlapped on each other, they reveal either a disequilibrium or equilibrium.  Fig. 7 on the left reveals a surplus disequilibrium. For example;

At P = 17,
Quantity demanded = 45 while
Quantity supplied = 57

A surplus of 12 results (or 45-57)

With a surplus, suppliers undercut each other pushing prices down towards equilibrium.