The numbers represent the ratio of nitrogen, phosperous and potassium. Theoretically, the nigher the nitrogen number, the faster the growth. So it appears that this type of fertilizer should be used when the plant is growing at its best.

Three months or so before a plant is due to bloom, the high nitrogen fertilizer should be stopped, as rapid growth is no longer desired, and the fertilizers with higher phosperous and/or potassium should be used to encourage bloom. That's why the fertilizers with low nitrogen and high phosperous/potassium are often called blossom boosters.