Form 656 is used when applying for an Offer in Compromise (OIC), which is an agreement between you and the IRS that settles your tax liabilities for less than the full amount that you owe.

The 2019 revision of Form 656-Booklet, Offer in Compromise (OIC) is available for download on IRS.gov, Monday, March 25. The booklet contains current forms and instructions for submitting an OIC. Using previous versions of the booklet may result in delayed processing of OIC applications.

Source: https://www.irs.gov