A more thorough explanation:

Definition: A parenthetical is a word or phrase that explains something that came before it. It is often enclosed in parentheses.

In legal writing, a parenthetical is a statement that can be added at the end of a source citation. This statement can explain why the source is relevant, how persuasive it is, or quote language from the source. However, parentheticals can be used in many other ways to help readers understand the text.

Example: The author argued that the new policy would be beneficial for the company's employees (who have been asking for better benefits for years).

In this example, the parenthetical phrase "who have been asking for better benefits for years" explains why the new policy would be beneficial for the employees.

Example: According to a recent study, the number of people who own smartphones has increased significantly in the past decade (Pew Research Center, 2019).

In this example, the parenthetical "(Pew Research Center, 2019)" provides the source of the information and gives readers more information about where to find the study.