It's the amount of money that your employer deducted from your income to send to the federal government or state for your income taxes. When you have more than the amount of your taxes taken out of your check, you get a refund. When you don't have enough money taken out of your paycheck for your taxes, you end up owing.
For more information, refer to the TurboTax article FICA and Withholding: Everything You Need to Know.