What the numbers mean
Every fertilizer bag should have 3 bold numbers. The first is the amount of nitrogen (N), the second the amount of phosphorus (P) and the third the amount of potassium (K). Nitrogen is in the form of ammonium (NH4) or nitrate (NO3). Phosphorus is in the form of phosphate (P2O5) and potassium is in the form of potash (K2O).
These numbers represent the percentage of each nutrient in a bag of fertilizer and are always in the same order - (N)(P)(K). A 20lb bag of 10-10-10 fertilizer contains 2lb (10%) nitrogen 2lb (10%) phosphorus and 2lb (10%) potassium. A 50lb bag of 18-24-6 starter fertilizer contains 9lb (18%) nitrogen 12lb (24%) phosphorus and 3lb (6%) potassium. Note that a fertilizer does not have to contain all three nutrients - urea (46-0-0) is an example.
What the nutrients do