Core aeration enhances oxygen levels in the soil and helps moisture and nutrients to enrich it.  It strengthens the roots, making the grass thicker and more resilient.  A core aerator is a machine that punctures holes into the soil and pulls out tiny plugs ½ to ¾ inch in diameter.  Because the cores are full of microorganisms that help decompose thatch, they are left on the surface to dissolve naturally.  Holes from aerating eventually fill in on their own.  Aeration vastly improves the quality of the soil and makes a much healthier lawn.