Alternative Minimum Tax (AMT) ensures that high income taxpayers who benefit from special tax deductions and credits pay at least a minimum amount of taxes.
Go to this section in Credit Karma Tax: Alternative Minimum Tax
There are certain types of income and deductions that are treated differently for AMT purposes than for regular tax purposes. Credit Karma Tax will calculate these based on the information you provide.
AMT is owed only if the tentative minimum tax is greater than the regular tax. Credit Karma Tax will help determine how much AMT, if any, you need to pay based on the information you provide in your tax return.
Source: irs.gov