The letter was sent to notify the insurer that Medicare has made payment on claims for which they may have primary payment responsibility.
What the letter means
The insurer should check its records and determine if they are the proper primary payer for the claims in question.
The insurer should refund Medicare the amount it paid on any claims they have primary payment responsibility for. If you have previously mailed a payment you will receive a letter acknowledging that payment has been applied to the debt and further advise you of any debt owed to Medicare. If a refund is due you will receive a check in a subsequent mailing.