Thatch is really just a build-up of dead grass. If it is too thick, it can block the absorption of water at one time of year, and create a breeding ground for lawn disease and insect pests in another. Water can accumulate in the thatch and grass roots can suffocate due to lack of air. If grass roots get entangled in thatch, they can become vulnerable to heat and drought.
The process of de-thatching is generally done with a machine although it can be done with a hard rake. Oakstead will make short work of thatching your lawns with a professional tool.