Goodwill usually stays constant on the balance sheet, so why would it be impaired, and what does goodwill impairment mean?

• It is written down when the acquired business's value falls below carrying value; a non-cash charge signaling the deal underperformed
• It is a cash payment to the seller
• Impairment means goodwill increased in value
• It happens only when the company is sold
• It is impaired every year on a fixed schedule