The tangled mat of organic matter around the base of the grass plant (above the soil) is called thatch. In small amounts it is good for the lawn. It acts as a natural mulch to retain moisture, lower soil temperature, cushions plants crowns, and discourages germination of weed seeds. The decaying plants add nutrients back into the soil.
Too much thatch can cause shallow roots, nutrient imbalance, and an invasion of insects and disease. When mowing turf with a heavy thatch, scalping may occur because the wheels sink into spongy turf. Grass which thick thatch is less tolerant to stress because of poor root structure.
Excess thatch results when the organic matter accumulates faster than the microorganisms can break it down. The could result from over-fertilization, infrequent watering, or leaving long clippings on the lawn.