The soil beneath the grass on your lawn tends to become compacted over time and as a result has too many solid particles in a certain volume or space which prevents proper circulation of air, water and nutrients within the soil. The reasons for this vary - lawns in family gardens get used for recreation rather than decoration, while in new build properties the lawn is often laid on nutritionally deficient subsoil rather than high-quality top soil. There may also be an issue with lawn thatch which is essentially strangling the grass and preventing it from growing properly.
In order to deal with this problem, lawns should be aerated twice a year, ideally in the spring as this is prime growing season and will allow time for the grass to recover. The practice of aerating a lawn involves perforating the soil with small holes to allow air, water and nutrients to penetrate the grass roots, resulting in a stronger, more vigorous lawn. Although possible to do this manually, it is more time-efficient and effective to either hire or buy a machine to do it. You will cover more ground evenly this way too...