What exactly is a mole of something?

The best way to think of a mole is the same way you would consider a dozen to be 12 but in this case a mole is 6.2 x 10²³ molecules. 

You can use moles to work out the mass of a substance needed or produced in a reaction if you know the reaction equation and a simple formula (moles = mass/molar mass (M_r)). 

The molar mass of an element or compound is simply the mass of 1 mole of it in grams! So for carbon, the molar mass is 12 therefore 1 mole of carbon weighs 12 grams. 

e.g. for the formation of water from OH^- and H⁺, one mole of hydroxide ions (OH^-) and one mole of hydrogen ions (H⁺) react to produce 1 mole of water:

OH^- + H⁺ ----> H₂O

...but hang on, how do 2 moles react to form only 1? Remember a mole is the number of molecules (6.2 x 10²³ to be precise) so when two molecules combine, they form one molecule so only one mole of water is produced. 

I hope that helps to simplify moles but the best way to get your head around them is by practicing mole-based questions!