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Simply put it is a ratio - the speed of the aircraft over the speed of sound. The Mach number signifies how many times the speed of sound you are flying. 

The speed of sound is 340 meters per second (1,126 ft/s). If a plane is flying that speed, it is flying Mach 1. If it is flying twice as fast, Mach 2, and so on.

Commercial airlines don't tend to fly faster than the speed of sound, but some jet planes can fly even Mach 10.