Chapter 3: Your Refundhttps://lukeko.com/14/chapter-3-your-refund 0
It's just the amount you paid (throughout the year before taxes are due) minus how much you actually owe. If the amount you paid is less then you don't get a refund -- you send money to the IRS.
The IRS likes to withhold part of your paycheck or get estimated taxes paid during the year since many people would spend all their money before they can pay taxes on it.
The withholding (and estimated taxes paid) is more than just your income bracket times your income. It also includes social security tax, medicare tax, and (if your state has it) state income tax