Have you ever driven by a yard that looked like it had little cylindrical plugs taken out of it and left all over the surface of the lawn? This is core aeration. This is best done in early spring every other year. This is important because it loosens up compacted soil and dethatches your yard.
Why is it important? Well if you have compacted soil then your turf-grass is not getting good root penetration and the absorption of adequate water and travel of adequate air supply is hindered resulting in a weaker more stressed lawn. It’s always a good thing to put down a fertilizer application right after core aerating your lawn to boost root growth and lateral movement.