A mole is like a dozen it is an arbitrary count of atoms, found in exactly 12 grams of carbon 12 or as originally defined as the number of atoms found in 22.4 liters of gas at STP
Carbon 12 is being used as a standard, to measure the number of atoms. There is a platinum bar in Paris which is the standard meter.
Carbon 12 has a a mass of 12 amu in one atom ( 6 protons and 6 neutrons. If you have exactly 12 grams of carbon Then 12 atomic mass units times the avogardo's number of atoms equals 12 grams.
This is important because no one has ever directly counted or measured Avogardo's number of molecules or atoms which is
This is important because Carbon 12 becomes a standard by which the number of atoms ( Avogardro's number) can be measured indirectly.
Exactly is important because even a small difference in the mass from 12 grams would result in a large difference in the number of atoms when you are dealing with number like