The three numbers on fertilizer represents the value of the three macro-nutrients used by plants. These macro-nutrients are nitrogen (N), phosphorus (P) and potassium (K) or NPK for short. The higher the number, the more concentrated the nutrient is in the fertilizer
Plants require nutrients in various amounts throughout their life-cycle. "Macronutrients" are what plants require more of throughout their life-cycle while "micro-nutrients" are needed in lesser amounts. As previously stated N.P.K. stands for Nitrogen, Phosphurus and Potassium. These 3 make up half of the macro-nutrients plants require in order to live with the other half being hydrogen, carbon and oxygen. Hydroponic nutrients do include micro-nutrients but the main focus is on macro nutrients.
Hydroponic nutrients bags or bottles will have 3 numbers printed on them usually in Bold. These numbers represent the percentage of each Macro-nutrient in the product. The same can be found on bags of soil fertilizer. For example, if the NPK ratio is 8-7-5, then the product contains 8% nitrogen, 7% phosphurus and 5% potassium and the rest is made up of micro-nutrients.