Deuteronomy 21:16 addresses a situation where a man has two wives, one loved and one hated, and each wife has sons. Even if the son of the favored, or beloved, wife is born after the son of the less favored, or hated, wife, the father cannot disregard the birth order and give the birthright inheritance to the son he favors.

Here's a breakdown:

"then it shall be, in the day that he causes his sons to inherit that which he has": This sets the context - when a father is dividing his inheritance among his sons.
"that he may not give the son of the beloved the rights of the firstborn": This is the core principle. The father cannot give the birthright inheritance (a double portion) to the son of the wife he loves more.
"before the son of the hated, who is the firstborn": This emphasizes the fact that even if the son of the hated wife is born first, his status as the firstborn must be respected.

In essence, the verse enforces the principle of birthright in inheritance matters. A father's personal feelings or preferences for one wife or son cannot override the legal right of the firstborn son, regardless of who his mother is.

Why is this important?

Protection for the disadvantaged: This law protected the son of the less favored wife, preventing him from being disinherited simply because his mother was not as loved.
Order and Justice: It established a clear and just system for inheritance, preventing favoritism from disrupting family harmony and potentially leading to conflict.
Firstborn's Responsibility: The firstborn had specific responsibilities within the family and community. Giving the birthright to a later-born son would disrupt the established social order.

The verse shows a concern for fairness and social stability, even in complex family situations.