Core Aeration is the best mechanical process you can do to your lawn. The aerator goes across the lawn and pulls plugs, or cores, between 2-3 inches. Those plugs look like goose poop and will naturally dissipate in a few days. This does several things. It de-compacts the soil allowing water and nutrients to get to the roots. Also it tears the root system of your lawn encouraging new growth. And finally the cores (goose poop) have bio-organisms in it that naturally break down your thatch layer on your lawn.
We recommend aerating at least once a year, generally in the fall. You can also throw down grass seed and fertilize around the same time. Consider aeration if your thatch layer is more than half an inch, you have heavy clay soil, erosion issues, puddling when it rains even a little, or if the grass is driven on or heavily used, causing it to look thin or unhealthy. In some cases it may even be beneficial to aerate in the spring and fall depending what you are looking to accomplish.