A more thorough explanation:

Definition: An innocent spouse is a person who may be relieved of liability for taxes on income that the other spouse did not include on a joint tax return. The innocent spouse must prove that the other spouse omitted the income, that the innocent spouse did not know and had no reason to know of the omission, and that it would be unfair under the circumstances to hold the innocent spouse liable.

Example: John and Jane are married and file a joint tax return. John fails to report all of his income on the tax return, resulting in an underpayment of taxes. If Jane can prove that she did not know about John's unreported income and had no reason to know, she may be considered an innocent spouse and relieved of liability for the underpayment of taxes.

This example illustrates how an innocent spouse can be relieved of liability for taxes that were not properly reported on a joint tax return. It also shows the criteria that must be met in order for a spouse to be considered innocent and eligible for relief.