I don't understand what exactly a mole is, could you please explain it to me?

Basicallly a mole refers to a number, in the same way as a dozen also refers to a number, this number being 12. Except instead of 12, the word mole refers to the number of 6 x 10²³, which is basically a 6 followed by 23 zeros. Therefore, in the same way as that you can have a dozen pears or apples, you can also have a mole of pears or apples (i.e.  6 x 10²³ pears or apples). In this way the word mole just means a specific number of things.