A mole is the Avogadro number of particles.. rather like a dozen is twelve of whatever, the mole is just that, an Avogadro number of whatever - I quite fancy a mole of pennies (or even better pounds!)!

The chemists define the Avogadro Number in terms of carbon 12 atoms: the number of carbon-12 atoms in 12 grams (0.012 kg) of unbound carbon-12 in its rest-energy electronic state.

The Avogadro number is a big number 6.02 x 10 ²³

It is given the symbol N_A (Number_Avogadro) by physicists and L (because in German scientific literature, sometimes also known as the Loschmidt constant) by chemists.
 
When we are dealing with atoms, having a large number like this to compute with is valuable.

The link between mass and number of particles is simple:

eg.

If you have 12g of carbon (atomic mass 12) there are 6.02 x 10 ²³ atoms of carbon in your sample. You have one mole.

If you have 2g of hydrogen (atomic mass 1, but diatomic molecules go round in pairs therefore the smallest unit has a molecular mass of 2) there are 6.02 x 10 ²³ molecules of hyrdogen in your sample. You have one mole.

If you have 18g of water (atomic mass of hydrogen is 1 and of oxygen is 16, therefore the molecular mass of H₂O is 18) there are 6.02 x 10 ²³ molecules of water in your sample. You have one mole.

Try the following questions:

How many atoms of lead are found in a 2mg sample ? (atomic mass of lead is 210g) ? Answer

What would be the mass of one hundred million million (100 000 000 000 000 = 10¹⁴) atoms of Uranium 238 (atomic mass is 238)?  Answer