The answer is: The amount of money that should be withheld in federal taxes
Most of employers is cooperating with the federal government to handle the tax payment of their employees.
To do this, the employers send out w-4 forms to be filled by each of their employees. The information in the w-4 help will be used by the IRS to determine the amount that should be withheld in federal tax. So every time you receive your salary, it would automatically be deducted with the tax payments.
You could also submit various information that can be considered as operational expense. This would reduce the amount of your tax and you can obtain tax refund from the IRS