Moles are the basic unit of measurement in chemistry. We use moles to compare amount of reactants and products in chemical reactions, to calculate the pressure and temperature in a balloon of gas, to determine the freezing point of a solution and to talk about how concentrated a solution is (for example, how salty the ocean is), among many other things. For this reason, understanding the mole is of vital importance if you want to succeed in chemistry.
To unerstand the mole, you should know a little bit about how it came about, specifically you should know a little about atomic masses and average atomic masses. You should understand Avaogadro's hypothesis and how it led to the mole. You should understand the how the mole relates to the mass of an element or compound with something we call molar mass. You should also understand how the mole relates to a number of particles with Avogadro's number. You should also know how the mole relates to the volume of a gas at standard temperature and pressure and to the concentration of a solution (molarity).
You will also need to be able to do math with the mole. Specifically, you will need to be able to convert between grams and moles, particles and moles, liters of gas and moles, and liters of solution, volume of solute and moles. You will also need to be able to relate all of these units in the same problem.
You should also be able to calculate the percent composition of a compound. In addition, you need to know what empirical and molecular formulas are and be able to determine the empirical formula of a compound from the percent composition and molecular formula from the empirical formula and the molar mass.