Aeration will poke holes in the grass pull out plugs of soil (like in this picture) and help the upward growth of the grass.

Healthy grass grows upright and is fluffy like this example in the picture.  Pictured next to the healthy grass is compacted grass that needs aeration.       

The top picture shows grass with mixed compacted areas and healthy, upright grass patches. The bottom pictures show the results after aerating the grass by making five passes in different directions with the aerator, following two weeks of growth.