A mole is a unit of measurement that is associated with the mass of a chemical substance. The mole is a specific measurement of the amount of atoms or molecules in a substance, based on the amount of atoms in 12 grams of carbon-12. The mole exists to give scientists an easy way to convert between grams and molecules and back again in chemistry.
The usage of the mole occurs when measuring the number of grams would not make much sense, but measuring down to exact numbers of atoms and molecules would not make sense either. The mole was created as a middle ground to measure a large number of atoms in a substance. It is not an exact measurement, merely an easy way to say that a substance has an approximate amount of atoms or molecules.
The mole was named as a unit of measurement in 1897 as a translation of the Mol, a word created by German chemist Wilhelm Ostwald and derived from the German word for the molecule. The concept of equivalent mass, a vital idea in the usage of the mole as a unit of measurement, was in use for about a century before this measurement came into usage.