A lawn aerator is a device that digs small holes in dirt with blades or spikes, breaking up dense soil to allow air, water, and plant systems to spread throughout the ground. Old fashioned aerators are hand-push machines with metal tines or saws on a rotating core. They operate by elbow grease alone. Modern versions may be powered by gas or battery engines.
In addition to breaking up soil, aerators tear apart thatch (dead organic material), improving the flow of air and water to the turf. By introducing these elements just beneath the surface of a lawn, aerators encourage worms and other small creatures to fertilize the soil where it's most useful to plants. They also separate the root systems of grass or other lawn cover, encouraging those roots to expand and grow thicker.
Most lawns only require a single aeration every year, so it may make sense to rent an aerator when you need it, rather than storing one for 364 days between applications. Lawns with more clay in their soil might benefit from two aeration sessions (one in spring and one in fall). In warmer climates, you can delay aeration until almost the beginning of summer. In cooler areas, aim closer to the start of spring.