That is the Recovery Rebate Credit if you did not get the 3rd Stimulus check. or if you qualify for more. Like if you had more dependents on your tax return. You had to enter the stimulus amount you already got so Turbo Tax can figure if you qualify for any more.
The 3rd Stimulus payment started going out in March 2021 and is for 1,400 for each person listed on your tax return, no age limit. The IRS used your 2020 (or 2019) return to send the checks out fast. If your 2021 return changes and you qualify for more you can claim the extra as the Recovery Rebate Credit on 1040 line 30. But line 30 is only if you didn't get the full amount or qualify for more. Not the amount you already got. The IRS also sent out letter 6419 to tell you how much they sent you . If you are married then each spouse should get a letter for half.