So, what actually is a mole?

A mole is simply a unit of measurement of any substance that contains as many atoms as there are in 12 grams of Carbon-12.

Why Carbon-12?

Carbon-12 is the standard by which we compare the mass of any other molecule however; it was not the first option. Originally Hydrogen-1 was used but this was soon discarded as they realized that because of its low mass it was very inaccurate. This is because Hydrogen-1 had a mass of 1.007825- you can imagine how hard it would be to calculate precisely every time! Whereas, Carbon-12 has a mass of 12.000000 so this allows for more precise answers when working out the mass of other molecules.