Refer to figure.
A supercritical wing is one that has been designed with specific streamwise segments to delay the onset of the adverse effects of the shockwave, i.e. wave drag and its severity and decreased lateral stability, when the airplane is flying in the transonic speed range or just above M_CRIT. It increases the cruise Mach number, the range, and the endurance of the airplane. The main features of a supercritical wing are:

The upper surface is flattened but the lower surface is reflex cambered towards the trailing edge. Thus, the airflow over the upper surface is not accelerated as much as conventional airfoils, which delays the formation of the shockwave to a higher speed.

Compared with a conventional airfoil section it has a larger leading-edge radius, a flatter upper surface and both positive and negative camber and enables a relatively thick wing to be used for approximately the same cruise Mach number.

The curved lower surface increases the lift generated by the aft end of the airfoil and replaces some of the lift lost from the flat upper surface. Because of the rear-loaded lift generation it is often called an aft-loaded wing.
The leading edge of the supercritical wing has a greater radius than traditional airfoils that suffer from excessive wave drag and the loss of longitudinal stability (tuck under).

By ensuring that its profile is carefully designed any expansion waves will be reflected from the subsonic boundary back to the shockwave, thereby weakening its intensity.