A tax credit you can take in advance (of determining your taxes for the year) to lower your monthly health insurance premium. When you apply for coverage in the Health Insurance Marketplace, you estimate your expected income for the year. If you qualify for a premium tax credit based on your estimate, you can use any amount of the credit in advance to lower your premium.
If at the end of the year you’ve over-estimated or taken more premium tax credit than you’re due based on your final income, you’ll have to pay back the excess when you file your federal tax return. If you’ve taken less than you qualify for, you’ll get the difference back.