This is my laymens view on IAS and Mach.
Imagine the air to full of particles. Lets say that at sea level the air is most dense. So 400mph (or knots) really does give you an IAS of 400mph.
The "speedomoter" on an aircraft measures the flow of these particles over the skin of the airframe.
As you get higher the air gets thinner.
So at 10000ft, going 400mph, you may only have 90% of the particles flowing over the airframe. This would give you an indicated airspeed of 360mph
At 35000ft the number of particles may only be 60% of those at sea level. So the same 400mph would give you an IAS of 240mph.
The speed at which you are travelling over the earth remains the same - you're doing 400mph - it's just that the "imaginary" particles are less dense the higher you go.
It therefore makes sense to measure speed in a different unit - this is known as Mach.
Hope this helps
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