Just to add to the above, the ratio that you've seen is N:P:K or in long hand, nitrogen (good for leafy crops): phosphorus (good for roots): potassium (for flowers and fruit).
If the ratios are different, you can tell which is the highest proportion of fertiliser, and therefore when to use it. So, for example, sulphate of potash (potassium) would be 0:0:x. Tomato fertilisers will always have the highest number in the K position.