This other income category is a "catch all" for all other ordinary income that is in AGI (adjusted gross income). This includes items on the first page of the standard 1040 (download PDF)
such as "business income," "farm income," "taxable interest income," dividends that are not "qualified," "alimony received," "taxable pensions," "unemployment compensation in AGI," rental real estate income, royalties income, partnership income, S-corp income, income derived from trusts, and more (such as gambling winnings).
In order to maximize accuracy, you should also reduce the amount you place in this "Other income" line by any adjustments, or "above the line deductions" that are on the bottom of the first page of a 1040, except the tuition and fees deduction, which should be included in the "tuition expenses" line of this calculator. For example, if you have self-employment income, you should manually calculate the self-employment tax you pay and take one-half of that amount as an adjustment.
The Tax Foundation sought to keep the calculator as simple as possible (i.e., contain has few fields as necessary to give a solid estimate), which is why this "catch all" category was added.