To aerify, a piece of machinery built for the task cores the ground (punches holes and removes the dirt) in a certain pattern. This is done to loosen soil that has been compacted by golfers walking over it, opening up growing room for the roots and increasing oxygen to the roots. It usually takes a couple weeks for the holes to fill in and grow over. Some courses charge a reduced fee during the aerification process - all should at least give you some warning of the condition of the greens.
For more information about the purpose and benefits of aeration, see the article "Aerification is a Necessary and Highly Beneficial Golf Course Practice."
Also see our Rules FAQ: "Do I get relief from aeration holes on the putting green?"
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