Aeration is essentially the process of "poking" holes in the ground.  What this does is

1. softens the ground to make it easier for roots to spread.
2. de-thatches overly thick areas
3. allows oxygen to enter the soil
4. the plugs that are removed decompose and make a good fertilizer
5. this should be done at least twice / year to promote a healthy lawn.
6. my aerations prices are very reasonable.