Form 6251 is used by individuals to figure their alternative minimum tax. Alternative Minimum Tax (AMT) is a separate method for determining income tax. The AMT is designed to provide a minimum amount of taxes paid by taxpayers with large tax deductions, exemptions, losses and credits. AMT is computed at rates of 26 and 28% of AMTI (Alternative Minimum Taxable Income) in excess of the exemption amount, depending on the total amount of AMTI.
The alternative minimum tax is a supplemental income tax imposed by the United States federal government required in addition to baseline income tax for certain individuals, corporations, estates, and trusts that have exemptions or special circumstances allowing for lower payments of standard income tax. Wikipedia