The critical Mach number of a wing is the speed, relative to the speed of sound, at which the flow over the top of the wing exceeds Mach 1. Since, according to the continuity equation, the flow over the top of a wing will be faster than the velocity of the aircraft, the critical Mach number is always less than 1.
A wing is typically intended for one of two conditions: subsonic or supersonic. Between the critical Mach number and Mach 1, the flow around the wing is both sub- and supersonic, which is an unstable and inefficient condition regarless of the type of wing, so these velocities are best avoided or hurried through.
The critical Mach number varies with wing geometry and angle of attack, but a number around 0.6 or 0.7 at 0 angle of attack is typical. Beyond the critical Mach number is the drag divergence Mach number, which is an undesirable operating condition for subsonic aircraft. Because the critical Mach number and the drag divergence Mach number are often fairly close, it is usually desirable to lower the critical Mach number by using swept wings.