TT asked you if you received the first two stimulus payments. If you answered NO to either or both, and it appears that you qualify, then it gave you a Recovery Rebate Credit for the missing amount.
$600 was the stimulus #2, which was sent out in late 2020/early 2021. The IRS is validating all of these credits against their records of those payments. They must also verify your income and dependency status to make sure you indeed qualify.
It's always, always a good idea to review your tax return before filing it.
