A more thorough explanation:

Definition: Certum an et quantum debeatur? is a Latin legal term that means "certain whether there is a debt due at all, and how much is owed." In other words, it refers to the two questions that must be answered before a defendant can make a plea in compensation.

Example: Let's say that John owes Jane some money, but they disagree on the amount. Before John can make a plea in compensation, the court must determine whether there is a debt due at all (certum an) and, if so, how much is owed (quantum debeatur).

Explanation: This example illustrates the two questions that must be resolved before a defendant can make a plea in compensation. The court must first determine whether there is a debt due at all, which means deciding whether John actually owes Jane any money. If the court determines that there is a debt due, it must then determine how much is owed, which means deciding on the specific amount that John owes Jane. Only after these two questions have been answered can John make a plea in compensation.