What box is that in or how is it listed?
Items of "Income" that are exempt from income tax can include amounts that you contribute to a retirement plan or a health savings account (elective deferrals), flexible spending account, or medical insurance premiums and certain other employee benefits that are deducted from your pay pre-tax. For example, if you are paid a gross salary of $40,000 but box 1 only shows $35,000 of wages, I would expect that you had $5000 of pre-tax deductions for these types of items.
Typically these could only be listed in box 14 which is an informal "information" box that does not directly impact your tax return.
You would have to ask your employer to really know what they are including in that box.
