According to the IRS, this code is simply a reference number that their telephone representatives use to help them research your account. If the IRS needs more information in order to process their returns, they will contact you, usually by mail.
The good news is that a 1121 message does not mean you are being audited. The bad news is there may be a delay in your refund while they sort it out. The IRS is stressing their Where's My Refund tool is the best method to check for any updates.
You may find more information by reading
this
Forbes article
.