As reported by KMBC News, you could be receiving what is called "Letter 6419" from the IRS starting by or through the end of January 2022.
This letter is associated with any advance child tax credit you might have received in the year 2021. If you receive this letter, it's advised you do not ignore it or throw it away thinking it's junk mail.
The letter will indicate the advance child tax credit payments you received in 2021 including the number of your children factored in to determine what your payments were.
The IRS is sending information letters to the 36 million families who received advance child tax credit payments last year. (Source.)
When you file your taxes for 2021, this letter will be needed for the information it contains, or maybe to accompany your taxes if you file in person.
It is advised that you treat this letter like it is an important tax form. In the past, any child tax credit available to you was calculated when you filed your federal tax return. In 2021, however, the American Rescue Plan Act provided three child tax credit changes.
Letter 6419 will be arriving in mailboxes through the end of January.
This letter could mean that the government owes you more money, or you may need to return some money to the government.
According to the IRS,
The IRS started issuing information letters to advance child tax credit recipients in December. Recipients of the third round of the Economic Impact Payments will begin receiving information letters at the end of January. Using the information in these letters when preparing a tax return can reduce errors and delays in processing.
People receiving these letters should keep them. Do not throw them away. These letters can help taxpayers, or their tax professional prepare their 2021 federal tax return.
More information regarding the Advance Child Tax Credit Payments in 2021 can be found here.