The numbers indicate the amounts or percentages of nitrogen (as N), phosphorus (as P2O5) and potassium (K2O) in the fertilizer. The first number refers to the amount of nitrogen, the second refers to the amount of phosphorus, and the third refers to the amount of potassium. For example, a 10-15-15 contains 10% N, 15% P2O5, and 15% K2O.