The stimulus checks many Americans received were an advance payment of a special 2020 tax credit, the recovery rebate credit. When you prepare your 2020 personal income tax return, you will need to reconcile the amount you received with the amount you were entitled to based on 2020 information. If you received too much, no need to pay it back.
Even better, if you did not receive some of the recovery rebate credit in the form of a stimulus payment, you will get to claim the additional credit on your 2020 tax return. For example, if your income for 2019 was above the threshold for the stimulus payment, but your income was down in 2020, you may qualify for additional credit. Also, if you had a baby in 2020, you are entitled to the credit for the new addition to the family.