I was just approved for SSDI I was wondering about the retro pay and how I need to take care of my taxes. I will be getting $31,208 in retro pay which is from December 2019 thru May 2021. I'm not sure how they came up with that because it doesn't add

Your total payments for 2021, including the retro pay, will be taxable on your tax return.  SSA will send you a statement at the end of the year showing your total payments, any taxes withheld, and other deductions to help you know how much to report on your tax return. 

The retro pay is known as a "Lump-Sum Payment" and while it is taxable, you have options on how to calculate the tax.  Rest assured that TurboTax will help walk you through these options when it is time to file your taxes.  In the meantime, if you would like to know more about the options, you can read about them in this article from the IRS.

I am not able to help you with how the retro pay was calculated though.  I hope knowing how it will be taxed is helpful.