A more thorough explanation:

Definition: Income in respect of a decedent. Income earned by a person but not collected before death. This income is included in the decedent's gross estate for estate-tax purposes. For income-tax purposes, it is taxed to the estate or, if the estate does not collect the income, it is taxed to the eventual recipient.

Example: If a person earned money from work before they died but did not receive it, that money is considered IRD. For example, if a person earned a paycheck in December but died before receiving it, that paycheck would be considered IRD. The paycheck would be included in the person's estate for estate-tax purposes and would be taxed to the estate or the eventual recipient for income-tax purposes.