What is a…
Hebrew: בְּכֹרָה —transliteration: bekorah
Meaning: the right of the first son born of the father; the firstborn son’s right
Male or female
In Old Testament Scripture, the terms “birthright” and “firstborn” do not apply to females, only males. They are male terms. Among the ancient Hebrews, the gender of the noun bekor translated in English as “firstborn” is male (masculine), not feminine; it always means the firstborn-SON. Since that was inherently understood in that culture, there is no need for the word בֵּן (ben) “son” or אּישׁ (ish) “man” (male) to be added to “bekor.”
Multiple wives situation
In the case where a man had 2 or more wives and potentially several sons from each, there was still only 1 firstborn son, and only 1 birthright. The eldest of all the FATHER’S sons had the birthright and the rank that flows from it, not the firstborn of the mother.
Also, biblically, a bastard son could not qualify as firstborn-son, only a legitimate son could do so, as this is a consecrated position/rank.
Privileges, advantages and responsibilities
Consecration to God / Priesthood
He became the priest of the family. Reuben, son of Jacob (Israel) was the firstborn of Israelite tribal patriarchs, and so the priesthood of the tribes belonged to him. That honor was, however, transferred by God from Reuben to Levi, as recorded in Numbers 3:12-13 and 8:18.
The firstborn son had allotted to him also a double portion of the paternal inheritance (Deuteronomy 21:15-17). Reuben was, because of his undutiful conduct, deprived of his birthright (Genesis 49:4; 1 Chr. 5:1). Esau transferred his birth-right to Jacob (Genesis 25:33).
If the father died or was badly disabled, the firstborn-son became responsible for protecting and providing for the family, when he was of age. He became the family’s replacement patriarch—the head of the family. He was responsible for making decisions and solving problems facing the his family group. He had to provide for his father’s widow (or widows), and for all his unmarried sisters.
The Jews attached a sacred importance to the rank of “first-born” and “first-begotten” as applied to the Messiah (Romans 8:29; Col. 1:18; Hebrews 1:4-6). As first-born he has an inheritance superior to his brethren, and is the alone true priest.