As its name suggests, the failure-to-pay penalty results when you fail to pay your taxes by the April tax return deadline.
The penalty amounts to 0.5% of your unpaid tax liability for each month or part of a month after the deadline that your taxes go unpaid. The penalty will not, however, exceed 25% of your total in unpaid taxes.
The IRS can also impose a failure-to-file penalty for failing to file your return on time. As the failure-to-file penalty tends to be harsher than the failure-to-pay penalty, it is best to file your return even if you cannot pay the tax due.
Note that even if you do request a 6 month extension to file, you are still responsible for paying at least 90% of your actual tax liability by the original deadline and then rest by the extended deadline. Otherwise, you will face a failure-to-pay penalty.