Thatch is a normal component in a dense vigorous lawn.
It is a tightly intermingled layer of living and dead plant material that accumulates between the layer of actively growing plants and the soil underneath.
Grass clippings left after proper mowing do not contribute to excess thatch thickness. Thatch that is not too thick can increase the resilience to heavy traffic.
It is not until thatch thickness increases to nearly 1 inch that it might compromise the ability to maintain a dense canopy of grass blades and vigorous growth.
Excess thatch thickness is most common on lawns that get more fertilizer and water than needed or when soils are compacted and have a pH outside the ideal 6.0 to 7.5 range.
Follow practices in four steps to success. Core aeration might also help relieve or minimize excess thatch. It is described in the soil compaction section.