If you are an individual who makes income from sources on which income taxes have not been withheld you may want to consider estimated tax payments. Tax authorities, such as the IRS and North Carolina, would like taxpayers to pay their income tax liability when the income is earned, not come tax filing time. As a result, they have setup estimated tax payment dates of 4/15, 6/15, 9/15, and 1/15. It seems like an odd set of dates- not evenly spaced- but I don't know the history of it.
How do you calculate your estimated tax payments? You need to roughly know what you will make for the year as well as your income tax withholdings for the year. Your income tax liability can be calculated and if you are short you should use estimated tax payments to cover that shortage. You can physically make payments via the EFTPS and NC DOR web sites. Note that for the EFTPS web site you need to register a week or two ahead of time.
Why do you need to pay estimated taxes? Because the IRS and states can penalize you if you substantially underpay your income taxes during the year. The underpayment calculation is complicated and something which you should discuss in regards to your situation with your CPA.