If you are checking the status of your Certified Mail letter, and you see the word "unclaimed", it means the intended recipient (who you were sending the letter to) didn't pick up the letter from the Post Office within the required 15 days.
When a mail carrier attempts to deliver a Certified Mail letter but no one is home, the mail carrier will leave a notice on the recipients door or put it in their mail box notifying the person that a Certified Mail letter is at the Post Office waiting for them to pick it up. This notice states, and USPS procedures dictate, that the letter will be returned to the sender, if it has not been picked up, after 15 days.
If you are the sender of the letter, you should receive it back within 10 days. Keep in mind, that in January of 2016 the USPS says that an undelivered Certified Mail letter is no long considered "Certified Mail" after it has been unclaimed or refused or is being returned to sender for any reason. This means that your letter will no longer be tracked by the USPS.