Guaranteed analysis
The numbers on a bag of fertilizer are called the “guaranteed analysis.” They represent the percentages of the three key ingredients in (most) fertilizers, which are referred to as NPK: the N is nitrogen, P is phosphorus, and K is potash1.
If you have a bag of fertilizer labeled 16-16-16, it would contain 16% nitrogen (for growth and green color), 16% phosphorus (for root development and flower/fruit production) and 16% potash (for plant health and foliage cell structure).
The guaranteed analysis varies from brand to brand and product to product, with different combinations of NPK used for different types of plants and lawns.
1 For those of you who want to know why K is for potash, “potash” is commonly used for the soil fertilizer forms of potassium, which has the chemical symbol “K” (from the Latin, which is kalium).