A more thorough explanation:

Equitable recoupment is a legal doctrine that allows a taxpayer to offset previously overpaid taxes against current taxes due, even if the taxpayer is no longer able to claim a refund on the previous taxes.

Let's say that in 2015, John overpaid his taxes by $5,000. However, he didn't realize this until after the statute of limitations for claiming a refund had passed. In 2019, John owes $10,000 in taxes. Under the doctrine of equitable recoupment, John can offset the $5,000 he overpaid in 2015 against the $10,000 he owes in 2019, reducing his tax liability to $5,000.

This doctrine is based on the principle of fairness. If a taxpayer has overpaid taxes in the past, it would be unfair to require them to pay additional taxes without allowing them to offset the overpayment. Equitable recoupment ensures that taxpayers are not penalized for mistakes made in the past.