Aeration is the process of breaking the surface of your lawn with a hollow metal tine(think cookie cutter) and pulling out a plug of soil. For established lawns with densely populated grass, this will allow oxygen to reach the roots and give more space for growth. For under performing lawns there are two primary benefits: 1. aeration creates a point of entry for fertilizer and lime to easily enter the soil root zone. 2. aeration creates surface area for seeds to adhere and germinate.
Aeration is a critical part of maintaining an organic lawn. Expect to aerate at least once a year, usually at the time of fertilizing and/or seeding.