Withholding is the equivalent of paying your eventual tax bill on layaway. Withholding is often a somewhat arbitrary amount of money that you give to the IRS to hold for you that they later give you credit for when it comes time to settle up by filing your tax return. It does not represent that actual tax liability for any particular portion of your income. Until you prepare your tax return, your actual tax liability is unknown. When you settle up, if you gave the IRS too much to hold for you, you get money back, a tax refund. If you gave them to little, you owe a balance due of the shortfall.
