The lookback rule is part of the Coronavirus tax relief package.
The special lookback rule will allow lower income individuals to use their earned income from 2019 to determine their Earned Income Tax Credit and the refundable portion of the Child Tax Credit, since their lower 2020 or 2021 income could reduce the amount they are eligible for.
You can look back and use your 2019 income for the EIC credit instead of 2020 & 2021. How to use the Lookback provision
https://ttlc.intuit.com/community/earned-income-credit/help/how-do-i-use-the-lookback-provision-to-m...
