Federal withholding is basically federal tax withheld from your pay by your employer (this will be on your W2). Since you mentioned that you received a refund for previous tax year, then chances are more likely than not you won't need to make estimated tax payments for the current tax year since you did not have a tax owed balance for previous tax year. However, if was that you owed taxes in the previous year, more likely that not, you would need to make estimated tax payments for current tax year, so you can hedge for not paying an underpayment penalty for taxes. Again whether or not and how much in quarterly payments to be made, will depend on your income circumstances for the current year, assuming the circumstances from the prior tax year still hold up for the current tax year plus the hypothetical (running your business bringing in less than $5K). Since, the hypothetical will be a self-employed/contractor-type activity, you would be responsible for making the taxes, thus your federal withholdings will be the estimated tax payments you would make quarterly.
