If you quit your job, and you had a 401k at that job, you have 3 options.
1. Leave the money in the account to grow tax-free (but, if the balance was small, they might force you out).
2. Rollover the money to a retirement plan at your new job or to your own IRA (tax-free rollover).
3. Withdraw the money and keep it.
If you withdraw the money and keep it, you will owe regular income tax plus a 10% penalty. Regular income tax might be 12%, 22%, or higher, depending on your other income, plus state taxes which can be between 3% and 12% depending on the state. Any withholding is just a mandatory estimate. The actual tax you owe is calculated at the end of the year on your tax return. You list all your income (including this withdrawal) and all your deductions, credits and dependents. Your tax is calculated and you get credit for any withholding (such as from paychecks and from this withdrawal). If you owe more tax than was withheld, you owe an additional payment, and if you had more withheld than your tax, you get a refund.
There is no way to know what your ultimate tax situation will be without knowing all your income and all your withholding. If you are single and your total income for the year is between $25,000 and $60,000, then the tax owed just on the retirement withdrawal will be 12% tax ($818) plus the 10% penalty ($682) plus any state income tax. So the withholding (by itself) is a little short. But whether this will add up to owing something or just getting a smaller refund will depend on your jobs, your dependents, and your other withholding.
