If you had health insurance from the marketplace, when you applied for the insurance, you gave healthcare.gov an estimate of how much income you would receive in 2024. They used that amount to calculate how much of the insurance premium would be covered by the tax credit and how much would be your amount to pay. So...you had some monthly amounts to pay, and the rest of the cost of having that insurance was paid by the government program.
If you ended up making a higher income than you told them you would receive, they re-calculate how much should have been paid by the program and how much should have been paid by you. And if your own share of the cost should have been more, they get it back on your income tax return.
If you actually had less income, you could even get some of the share you paid back on your tax return.
From Healthcare.gov:
If you're enrolled in a Marketplace plan and your income or household changes, update your application as soon as possible.
These changes — like higher or lower income, adding or losing household members, or getting offers of other health coverage — may affect the coverage or savings you’re eligible for. After you finish applying or enrolling, you may be asked to submit documents to confirm your income.
Discover which changes to report.