The use of sweep back reduces the effective velocity at the leading edge. Since the velocity component parallel to the leading edge does not contribute to the change in aerodynamic behavior, the critical Mach number is reduced to the value M’=M/cos (Busemann’s independence principle).
Figure 1: swept back wing
This technical solution not only delays the transonic drag rise, but it also reduces the rate at which the drag increases in the transonic regime, as shown in the following figure.
A similar effect ca be achieved with a forward sweep, although there appear some stability problems with consequent difficulty in maneuvering the aircraft.
While the back sweep is ideal at transonic and low supersonic speeds, it does have some drawbacks at low speeds: high induced drag and loss in lift coefficient. To fly at both regimes wings of variable sweep may be required (these wings, though, have their own problems).