If you have underpaid your taxes or you haven't paid them on time what you can expect is to get penalized by the IRS. They will apply a penalty interest rate for the late period and also charge you some fees. The penalty amount will get capitalized in your outstanding taxes and grow over time making it even harder to pay out your taxes.
As the IRS is not interested in you not paying your taxes they have devised the process of penalty abatement. The assumption is that there are cases where the tax payers can pay their taxes in full if they are relieved from the penalties due. In such cases they can submitting a request for penalty abatement which would generally be approved if they can not be blamed for their lateness or underpayment.
Sos in general tax penalty abatement is a method by which a taxpayer can receive forgiveness for part or all of the money that they owe the IRS. This option however applies only to specific situations, such as the death of a member of the taxpayer's immediate family, the destruction of a building in which the taxpayer's records are stored, or a delay by the IRS in printing or mailing tax documents.