Topdressing is a four process system that involves putting nutrient rich soil on the top layer of surface to improve lawn appearance and thicken turf.  The four steps are:

Aeration    Creates holes(plugs) to allow nutrient rich soil to fill in (during warm months)

Applying nutrient rich and organic topsoil

Fertilizer  to stabilize PH of the soil level

Reseeding  To thicken grass