Core aeration simply put is where hollow tines are used to puncture holes in the lawns surface which then pull out plugs of soil.  Your grass, just like us, has the need to breathe.  Aeration promotes stronger, deeper and more vigorous root growth as it brings more oxygen down into the soil.  It alleviates compaction which reduces the amount of nutrients and water that can penetrate the soil.  This process is one of the best ways to improve the quality and lushness of your turf.