This event, which occurs twice a year (once in the spring and once in the fall) is met with distain from golfers as the greens become slow, messy and tough to play; However, this process is absolutely necessary for the health of the greens.
Aeration is a process in which a machine takes small holes out of the grass that exposes the roots of the soil to air, water, and nutrients, as well as giving space for the grass to grow without "clumping" and provides in return the smooth, quick greens that golfers love. Without aeration, the greens would become tough, bumpy and wouldn't have much resilience, taking much longer to heal after any potential damage that may occur.
For more information on green aeration, see here. http://golf.about.com/od/golfcourses/a/aerification.htm