The CARES Act provides that NOLs arising in a tax year beginning after Dec. 31, 2017 and before Jan. 1, 2021 can be carried back to each of the five tax years preceding the tax year of such loss. 
So, an NOL on a 2018, 2019 or 2020 tax return could now be carried back and create a refund in of taxes paid in prior years.
The Act also temporarily restores the taxable income limitation to allow an NOL carryforward to fully offset income. For tax years beginning before 2021, taxpayers can take an NOL deduction equal to 100% of taxable income (rather than the 80% limitation in present law). For tax years beginning after 2021, taxpayers will be able to take: (1) a 100% deduction of NOLs arising in tax years prior to 2018, and (2) a deduction limited to 80% of modified taxable income for NOLs arising in tax years after 2017.
Those with Net Operation Losses in 2018 and 2019 should contact their tax professional to find out if a tax refund can be created for prior years by carrying these losses back to prior profitable years.