It is called the Collection Statute Expiration Date (CSED). That is the expiration of time period established by law that the IRS has to collect a given tax. Once a CSED expires the IRS will write it off.
Assume that you filed your 2017 form 1040 on time on April 18, 2018. You owed taxes for that return that you did not pay. All things being equal, after April 19, 2028 the IRS can no longer lawfully collect that tax and you are off the hook.
A CSED starts only after a tax return has been filed. The CSED does not start automatically on the due date. If a taxpayer filed their 2017 tax return on June 1 of 2020 that is when their 2017 CSED will start. If there are no tolls on the CSED, on June 2 of 2030 the IRS can no longer legally come after that 2017 tax.
The CSED can be tolled, or stopped, by different collections and legal actions, so it is not always easily defined. Because a serious IRS collections matter will take years to resolve, and the taxpayer and their representatives will try different strategies for relief, it is typical for a CSED to be tolled and re-computed. Some actions that toll a CSED are: Collection Due Process Cases, being in a Disaster Area, Military Postponement, Bankruptcy, Offer-in-Compromise, Installment Payment Agreement, Summons Enforcement, Taxpayer Assistance Order, and Innocent Spouse. When a taxpayer is in any of these situations, the CSED changes and will need to be re-computed.
Sometimes a taxpayer owes more than one type of tax, and for more than one period; businesses have multiple due dates throughout a given year. You can imagine how complicated calculating any CSED can get when a taxpayer falls a couple of years behind. You will need the help of a licensed professional if you plan to use the CSED as part of a larger strategy.