A NOL means you have a business or rental loss that results in negative income (Line 41 of your Form 1040).
Yes, I am sure about the NOL. For 2017 tax returns, it can be carried back for two years, and/or it can be carried to the next year (up to 20 years forward (for future readers, starting in 2018, NOLs can usually no longer be carried back).
As for where in TurboTax, I have no idea. Have you gone through the entire interview? It should be near the end (it won't know you have negative income until after you have gone through all income and deductions).
