Plain English Meaning

Mach number tells you how fast you're going compared to the speed of sound. Airspeed tells you how fast the air is flowing past the aircraft in knots. They are not the same thing, and the relationship between them changes with altitude and temperature. High up where the air is cold, the speed of sound is slower, so the same airspeed gives you a higher Mach number.

Full Definition

A concept describing the distinction between Mach number, which expresses an aircraft's speed as a ratio to the local speed of sound, and airspeed, which expresses speed in knots through the surrounding air. At high altitudes, the speed of sound decreases as air temperature drops, so a constant indicated airspeed corresponds to a higher Mach number as altitude increases. Conversely, holding a constant Mach number while descending into warmer air results in an increasing indicated airspeed. Pilots of high-altitude, high-speed aircraft must because limits at altitude are governed by Mach number, while and handling limits at lower are governed by airspeed.