The IRS has been told by the Federal Marketplace (or a state marketplace) that you had insurance coverage under the Affordable Care Act ("Obamacare").

Taxpayers who enrolled in this insurance typically received Premium Tax Credits based on your estimated income for the year (which you made when you enrolled at the beginning of the year). These tax credits were paid directly to the insurance company to reduce your premiums.

At the end of the year, you are required to file form 8962 with your tax return to determine if the amount of tax credit paid during the year was appropriate for the amount of income that you eventually reported at the end of the year. If you were paid too little, then you get a credit on your return; if too much, you are charged for the overpaid credit.

If you do not file the 8962 in a timely manner, the IRS is likely to assume that you were not due any Premium Tax Credit at all, and will send you a letter stating that you owe X dollars along with penalties and interest.

It is definitely in your interest to respond to that letter in a timely manner.

[Edited 4/2/2020 1:54 pm CDT - typos]