This term refers to the ratio of nitrogen (N), phosphorus (P), and potassium (K) in fertilizers.
It is always expressed as N-P-K.
For example, a fertilizer labeled as 24-6-6 has 24% nitrogen, 6% phosphorus, and 6% potassium
In this example, the remaining 64% of the fertilizer is comprised of inert matter.
Remembering which nutrient does what is easy if you'll remember this simple phrase:
"up, down, all around"
up: Nitrogen promotes top growth
down: Phosphorus promotes good roots
all around: Potassium benefits the whole plant