President Biden just signed the American Rescue Plan Act of 2021, which will temporarily increase the the child tax credit from $2,000 per child under 17 to $3,000 ($3,600 per child under age 6). This tax credit will also be fully refundable.
If you qualify you can get an advance from the IRS, where they will send periodic payments to families from July 2021 to December 2021.
The increased tax credit will start to phase out at AGIs of $75,000 on single returns, $112,500 for head-of-household returns, and $150,000 on joint returns. Families who aren't eligible can still claim the credit of $2,000 per child.
If you don't opt out of the advance payments the IRS would send out a check from July to December and would account for half of your 2021 child tax credit. The monthly payments would be up to $250 per child ($300 per child under age 6). You would then claim the rest of the credit, when you file your 2021 tax return.
To determine if you are eligible for advanced payments the IRS will first look at your 2020 return. However, if you haven't filed one yet, the IRS would look at your 2019 returns.