The Bible Says
Numbers 3:1-4 Meaning

Now that the twelve tribes appointed to serve a military function have been counted and organized around the tabernacle, Moses then turned to the role(s) of the tribe of Levi. He started with the records of the generations of Aaron and Moses. Aaron is the brother of Moses, and both are from the tribe of Levi, who was the fourth son of Jacob, from his wife Leah. The word generations (Heb. "toledot") is familiar in the books of Moses, especially Genesis (Genesis 2:4, 5:1, 6:9, 10:1, 11:10, 11:27, 25:19 et al.). In other English versions, it is translated "records" (NKJV), "account" (NIV), or "lineage" (NRSV). In Genesis, it is used to begin a new section of the book containing an account of a family lineage. Here, it concerns the family of Aaron and Moses (the Levites) being dedicated to serving the LORD in the tabernacle.

These instructions were given to Moses at the time when the Lord spoke with Moses on Mount Sinai. They involved the sons of Aaron, who were Nadab ["generous, noble"] the firstborn, and Abihu ["he is father"], Eleazar ["God has helped"] and Ithamar ["land of palms"]. This repeats what was stated in Exodus 28 - 29, that these are the names of the sons of Aaron, the anointed priests, whom he ordained to serve as priests. Aaron and his descendants were to be set aside to minister in the LORD's presence.

By the time the Israelites reached the plains of Moab, on the eastern border of the Promised Land, and prepared to cross the Jordan River and enter the land, there were only two of Aaron's four sons remaining alive. The reason is that Nadab and Abihu died before the Lord when they offered strange fire before the Lord in the wilderness of Sinai. The story of their demise can be found in Leviticus 10:1 - 2. They died because they did not limit their activities to what the LORD commanded. They offered "strange fire" (possibly an unauthorized incense offering). They did "what the LORD had not commanded them" (Leviticus 10:2). They apparently took matters into their own hands to prescribe worship, rather than strictly obeying the LORD. The LORD killed them as a result. Plus, they had no children, so there would be no descendants carrying on their line. As a result, Aaron's remaining sons Eleazar and Ithamar served as priests in the lifetime of their father Aaron.