Aeration is the technical term used for adding air gaps to soil. Usually produced by use of mechanical equipment that either punctures the soil with spikes or removes cores of soil from the ground.
Aeration is used in turf to reduce compaction and thatch buildup, which allows air, water and nutrients to reach the roots. This allows the roots to spread and grow less dense which promotes a healthier, greener lawn.
If you have clay type soil you should aerate twice a year (Spring & Fall) If you have more sand in your soil then once a year should be good (Spring or Fall).