A partnership or S Corporation has listed you as a partner or shareholder receiving Illinois source income. The partnership or S Corporation should have provided you with an Illinois Schedule K-1-P, Partner's or Shareholder's Share of Income, Deduction, Credits, and Recapture. This income is taxable to the state of Illinois. If you did not receive this information, you must go back to the partnership or S Corporation for the Illinois K-1-P.
For more information regarding pass-through income, please see Publication 129, Pass-through Entity Income.