Unrelated business taxable income is the income produced by tax-exempt organizations through activities that are not related to the organization's primary tax-exempt purposes.
For example, if a school generates income through activities that are not related to an educational purpose, the income will be considered as an unrelated business taxable income.
An organization has to file a Form 990-T to if the gross unrelated business taxable income of the organization is $1000 or more and if an organization fails to do so, it has to provide the IRS with a reasonable explanation on an attached Schedule O.