It is the pin Illinois uses to verify your identity when filing your tax return. 

If you don’t know your Illinois PIN you can look it up on Illinois’ website (http://www.revenue.state.il.us/ElectronicServices/Individuals/) or by calling (800) 732-8866 or (217) 782-3336. To get your PIN, you will need your Social Security Number and one of the following:

1.    2016 Adjusted Gross Income (IL-1040, line 1)
2.   Drivers’ license number
3.   Illinois state ID number

For more information on signing your IL return, see the TurboTax FAQ below:

    Before you can e-file your Illinois taxes, Illinois requires that you provide some personal information to confirm your identity. They’ll compare this info with their records before they accept your tax return. If the information doesn’t match, there may be delays processing your taxes.