Over time, little bits and pieces of grass die and gather just above the soil. This is called thatch. A little bit of thatch can be beneficial. It's organic material that is broken down by microbes in the soil. But sometimes, thatch builds up too fast for natural processes to break it down. It forms a barrier, keeping moisture and air from going where your grass needs it. A half-inch or more of thatch can weaken your lawn. You have 2 ways to remove it: dethatching and aerating.
If you have a small patch of lawn and don't mind a little exercise, you can use a convex rake. As you pull it through the grass, it lifts up and moves the thatch. Chances are, you'd prefer a power rake, which can be rented at some hardware stores or equipment rental centers. It combs your grass and kicks out a large amount of debris, which you'll have to rake off afterwards. If you live in a warm climate, be sure to ask if the machine you're renting is suitable for your type of grass.
For cool-season grasses, the best time to dethatch is late summer or early fall. That's when your grass is growing most vigorously. For warm-season grasses, dethatch in late spring or after the second mowing. Before dethatching, mow a little shorter than normal. Also, you'll achieve better results if you do it after a light rain or watering.
Aerating, coring, and aerifying are different terms you might hear for the same procedure. An aerator pulls plugs out of your soil. This helps loosen compacted soil and allows air and water to reach the roots. Aerating is considered a better way to dethatch your lawn since it puts less stress on it.
As with dethatching, you want to aerate when your grass is at its peak growing season - late summer or early fall for cool-season grasses, or late spring for warm-season grasses.
If you have high-traffic areas and heavy thatch, you might want to aerate every year or 2. If your lawn looks great, you can cut back to every 3-5 years.