A more thorough explanation:

A prohibited degree refers to a degree of relationship that is so close that marriage between the individuals is forbidden by law. This includes relationships like brother and sister, aunt and nephew, and uncle and niece. The law generally forbids marriages between all persons who are lineally related and within the third civil-law degree of relationship.

For example, if a man wants to marry his niece, it would be considered a prohibited degree of relationship and would be illegal.

The term "prohibited degree" comes from the Bible, where the incest prohibition is pronounced in Leviticus 18:6-18. In addition to being illegal, these relationships are also considered taboo in many cultures.

It's important to note that prohibited degrees are used not only to determine who is the closest heir but also to establish the incest prohibition in marriage requirements.

For instance, if a woman's father dies without a will, his estate will pass to his closest kin, counting degrees of kinship. To calculate the degree of relationship of the woman to the estate, one counts the steps (one for each generation) up from the father to the nearest common ancestor of the father and the woman, and on down to the woman from the common ancestor. The total number of steps is the degree of relationship.

Overall, prohibited degrees are a way to prevent incestuous relationships and maintain the integrity of familial relationships.