What is a mole?

A mole is a measure of quantity, such that when you have a mole of something, you have 6.02 x 10²³ atoms/molecules of it. The number of atoms/molecules in a mole is called "Avogadro's Number" and is the number of atoms in 12g of carbon-12. For example, if I had 12 eggs in a box, you could say I have a dozen eggs in the box. Similarly, if I have 6.023 x 10²³ molecules of water in a box, you could say I have a mole of water in the box.