If you have ever seen the little soil plugs and holes all over a lawn, you have seen the results of core aeration. A spiked drum or reciprocating punch will pull these cores out and deposit them on your lawn. Aerating is done to reduce soil compaction. By punching those holes out, the aerator is making way for air, water and nutrients to get to the lawns root system. The holes also create space for the soil to expand. Compacted soil is tough for grass to grow in, but ideal for weeds.
We recommend aerating your lawn in fall. In the following spring, the holes will benefit from the snow runoff and the plugs will break down and redeposit their nutrients into the lawn. Aerating also helps the lawn absorb any fall fertilizer you put down.
Here is a great video about core aeration: Aerating