Thatch is essentially a layer of partially decomposed stems, rhizomes and roots that forms a brownish mat on the soil surface. It is not caused by lawn clippings left on the lawn, because they are very high in water content and break down rapidly.
A thin thatch layer acts as a sort of cushion by absorbing the weight of foot traffic, holds water and protects the soil from sudden temperature changes. A thick (more than 1 cm) layer of thatch, however, can prevent water and air from reaching the soil. It also tends to harbour some kinds of insects and promote disease.
Mechanical dethatchers with vertical blades are often recommended. After dethatching, the lawn should be aerated, reseeded and topdressed, to encourage it to grow back quickly before weeds can take over. Dethatching is very labour intensive, however, and badly damages a lawn’s root system. For this reason, it is best to encourage thatch to decompose gradually by aerating, reseeding and topdressing your lawn. With organic lawn care, once you have renovated your lawn, thatch shouldn’t be a problem anymore.