Receiving a lump sum SS "catch up" payment for a previous year can some times push a tax payer into a higher tax bracket. For this reason, you allowed to do an alternate tax calculation that will tax the lump sum as if it had been received the previous year. TurboTax can do the calculation and compare it to the current year calculation and recommend a choice for you. You will have to have last year's tax info handy when you ask TT to do the alternate calc.
You can choose to just skip it. But if last year's income was less than this year, you will probably come out better with the alternate.