Lawns will eventually get clogged up with dead and decaying organic matter (thatch), which will inevitably prevent vital water and nutrients getting through to the roots.
Scarification is the removal of thatch from the surface of the lawn using a mechanical scarifier. An added bonus of this is that it also provides an element of aeration, improving drainage and disease tolerance. The thatch is then raked out of the lawn and bagged for your disposal.
Scarification should only be carried out when grass is actively growing to allow it time to recover from the stress. Ideally this should take place annually either in Spring or Autumn and can be followed up by over-seeding. Not only will over-seeding remedy tired, thin parts of your lawn, it also allows the opportunity to introduce stronger, drought resistant and shade tolerant varieties into your lawn.