Starter fertilizer is used when a new lawn is planted. It has a large 2nd number like 20 - 25 - 10. Unless you are planting seeds, do not use starter fertilizer. Do get your soil tested to determine yoursoil's PH, Phosphorus and Potassium needs. Autumn is the correct time to fertilize, early September, late November. Many Soils in the midwest need little if any Phosphorus.

I AM planting seeds - or did plant them a month ago!

HOWEVER - now that you mention it what DO those numbers mean on fertilizer

Usually, there are three numbers on the bag. They're often referred to as NPK, because they stand for Nitrogen, Phosphorous and Potash (the K comes from the symbol for element Potassium in potash).

The number shows the percentage of each in the bag, so you can determine how much you're adding to the soil by multiplying by the number of pounds of fertilzer and dividing by 100. So, if you have a 20 lb bag of 10-10-10, you're applying 2 lbs each of nitrogen, phosphorous and potash.

Sometimes the fertilizer will also have information about other nutrients it has, such as iron or sulfur, but the only ones that are required are the NPK.

I AM planting seeds - or did plant them a month ago!

You did? A cool season grass I assume? Be prepared to do it all over again in about 6 to 8 weeks.