What is underpayment of estimated income tax? and why am I being charged a fee?

[LinaJ2018]

If you did not pay enough taxes during the year, either through withholding or making estimated tax payment, you might be penalized by the government and that's probably what the additional fee is for. Your penalty is calculated on a Form 2210 https://www.irs.gov/pub/irs-pdf/f2210.pdf

Generally, you will be penalized if you either owe more than $1,000 in tax after subtracting their withholding and estimated tax payments, or if you did not pay 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.

For more information, see:

https://www.irs.gov/taxtopics/tc306.html