That would be the amount of the credit reduced because of income.
That is calculated on Schedule 8812 which is included with your tax forms.
According to the IRS:
A4. The Child Tax Credit begins to be reduced to $2,000 per child if your modified AGI in 2021 exceeds:
$150,000 if married and filing a joint return or if filing as a qualifying widow or widower;
$112,500 if filing as head of household; or
$75,000 if you are a single filer or are married and filing a separate return.
The first phaseout reduces the Child Tax Credit by $50 for each $1,000 (or fraction thereof) by which your modified AGI exceeds the income threshold described above that is applicable to you.
IRS Link