Keeping it very simple
The airspeed indicator works by picking up the increased pressure as the airplane travels through the air and displays that information as an airspeed.
But the atmosphere is not a constant density, the higher one goes up in the atmosphere the less dense the air is (thinner). So the basic airspeed indicator cannot compensate for the significant differences in density at the higher altitudes. What worked wonderfully at sea level and at 1,000 meters, does not work so well at 10,000 meters.
We need a new reference point from which to measure our relative speed. The speed of sound works well in this regime. So we transition from IAS (indicated airspeed) to Mach numbers.