"FIT" on a pay stub stands for federal income tax. This is the amount of money an employer needs to withhold from an employee's income in order to pay taxes.
In the United States, federal income tax is determined by the Internal Revenue Service. FIT is applied to taxpayers for all of their taxable income during the year. The rate is not the same for every taxpayer. Some entities, such as corporations and trusts, are able to modify their rate through deductions and credits. Other groups, such as charitable organizations, can apply for tax-exempt status.
Some states also have an income tax of their own. On a pay stub, this tax is abbreviated SIT, which stands for state income tax.