Eligibility for premium tax credits is based on your Modified Adjusted Gross Income, or MAGI. When you file a federal income tax return, you must report your adjusted gross income (which includes wages and salaries, interest and dividends, unemployment benefits, and several other sources of income). MAGI modifies your adjusted gross income by adding to it any non-taxable Social Security benefits you receive, any tax-exempt interest you earn, and any foreign income you earned that was excluded from your income for tax purposes.
Note that eligibility for Medicaid and CHIP is also based on MAGI, although some additional modifications may be made in determining eligibility for these programs. Contact your marketplace or your state Medicaid program for more information. (26 C.F.R. § 1.36B-1; IRS, Questions and Answers on the Premium Tax Credit).