Aeration is the act of breaking apart compacted soil that blocks nutrients and stunts root growth.
Lawns should be aerated once a year. In cooler regions, aerate in early fall or early spring. In warmer regions, aerate in early spring or before summer. Without regular aeration, grass will thin out and die; aeration is especially important for newly planted sod, heavily trafficked lawns and new constructions. For maximum impact, aerate your lawn before fertilizing it. That way, the fertilizer can get through to the grass roots.
For hard soil or lawns with water runoff, use a plug aerator. Spike aerators work better on looser soil.
After aerating your lawn, let any excess soil dry and break down, then overseed or fertilize the lawn as needed.