Aeration is the process of removing sections of soil, called plugs, to help oxygen, water and nutrients reach the root zone of turfgrass. This is a common solution to correct compacted soils, which occurs when soil becomes so packed down that it is hard and impenetrable. Compacted soils typically inhibit air and water distribution to the root zone of your grass, which can lead to dead patches of lawn.
You should aerate a lawn that is having trouble taking in soil and water. The best times to aerate are when it is warm but not hot and when the soil is moist but not soggy.