The maintenance practice of punching holes in the turf to relieve compaction, manage thatch, and let roots breathe — the reason your favorite green looks freckled a couple of times a year. The most common form, core aeration, physically pulls out small plugs of soil, often around a half-inch across, then the holes are backfilled with sand. That sand dilutes the spongy layer of decaying roots and stems, opens channels for water and air, and firms the surface back up. Greens take the most traffic of any surface on the course, so this reset is essential; putts are bumpy for a week or two, but skipping it trades short-term smoothness for long-term decline.