This is not a bill - the IRS has made changes you requested to your tax return and there was an overpayment based on this adjustment. You will be receiving an additional tax refund within 2-3 weeks of the notice.
The notice explains the changes to the tax return.
If you agree with the adjustment(s) make the appropriate changes to your tax return record. If you do not agree with the change, call or write to the IRS at the phone number or address given in the header section of the notice.
Sections of the Notice:
Sample CP 21B: PDF Pages 1-2
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.