The definition of mole of a substance

The mole is the SI unit for the amount of substance. It is defined as the amount of substance that contains exactly $6.02214076 \times 10^{23}$ elementary entities (such as atoms, molecules, ions, or electrons). This specific number is known as Avogadro's number ($N_A$).

Essentially, a mole provides a standard way to count a very large number of particles and relates this count to the macroscopic mass of a substance through its molar mass. The molar mass of a substance (in grams per mole) is numerically equal to its atomic or molecular weight.