A Net Operating Loss or NOL is a period of time during which the tax-deductible expenses of a company are greater that the total income of the company. This means that the company is essentially operating at a loss and is therefore required to pay less tax.
In order to remain fair, companies are taxed when they are profitable and should therefore receive tax money back when they are unprofitable. This happens in the US in the form of NOL tax relief. When a company posts a, they can either claim tax credits back from previous amounts of tax they have paid, or they can use the loss to offset future tax liabilities.