The Thatch Problem
As yearly growths of grass live and die, grass roots can get tangled with dead grass organic matter into a thick, matted layer called thatch. Thatch stops new grass roots from penetrating deeply into the soil and damages soil drainage as well. If the lawn gets a lot of foot traffic, it can become compacted, further weakening the grass. The grass becomes less healthy and attractive. It also becomes more vulnerable to weeds such as dandelions and thistles, as well as insect infestations.
Aerators breaks up thatch and loosens the soil to allow the roots to spread out and water to drain properly. There are two basic types of lawn aerators: core and spike. Core aerators actually scoop tiny plugs of lawn out as they go, creating gaps in the soil. Spike aerators shove spikes into the soil, creating holes. These spikes can actually compact the soil more, so core aerators are usually recommended as the preferred method.
A lawn aerator has a rotating drum with either spikes or spoons sticking out of it. As it turns, it mechanically makes holes in the lawn. There are also aerator attachments that can be dragged behind lawnmowers, and work in the same way. One more novel approach is aerator spike shoes. These shoes, looking like a more menacing version of soccer cleats, make holes in the lawn as the wearer walks. Because they compact the soil and are hard to use evenly over the whole lawn, however, they are not generally recommended by lawn experts.