What do the numbers on the fertilizer mean?

The 3 numbers (12-6-8) represent the value of the 3 macro-nutrients used by plants.

They provide nutrients UP, DOWN and AROUND the plant.
12= NITROGEN- Largely responsible for the growth of the leaves= UP.
6= PHOSPHORUS- Largely responsible for the root growth of the plant= DOWN.
8= POTASSIUM- Nutrient that helps the overall functions of the plant= AROUND.