If you do not pay enough tax, you may have to pay a penalty for underpayment of estimated tax. Even if you are getting a refund you can still owe a penalty for not paying in evenly during the year. Generally, most taxpayers will avoid this penalty if they owe less than $1,000 in tax after subtracting their withholdings and credits, or if they paid at least 90% of the tax for the current year, or 100% of the tax shown on the return for the prior year, whichever is smaller. It is included in your tax due or reduces your refund.
FAQ https://ttlc.intuit.com/community/tax-payments/help/how-do-i-add-form-2210/00/25703
Getting a Refund and have a penalty?
https://ttlc.intuit.com/questions/2584221-why-am-i-getting-an-underpayment-penalty-if-i-m-getting-a-...
IRS info on Underpayment Penalties..
http://www.irs.gov/taxtopics/tc306.html#skiptocontent
