"3 These are the generations of Aaron and Moses at the time when the Lord spoke with Moses on Mount Sinai.2 These are the names of the sons of Aaron: Nadab the firstborn, and Abihu, Eleazar, and Ithamar. 3 These are the names of the sons of Aaron, the anointed priests, whom he ordained to serve as priests. 4 But Nadab and Abihu died before the Lord when they offered unauthorized fire before the Lord in the wilderness of Sinai, and they had no children. So Eleazar and Ithamar served as priests in the lifetime of Aaron their father."
Numbers 3:1-4
The next two chapters describe the special position of the Levites. In 3:151 the nature of their calling and the purpose of their being set apart are described, while chapter 4 delineates the service that the various branches of the tribe were to perform.
These introductory verses about the sons of Aaron serve to explain why Eleazer and Ithamar, Aaron's younger sons, figure in the narrative of Numbers rather than Nadab, his first- born, and Abihu, his second son. Nadab and Abihu, we are told in 4, 'died before the Lord, when they offered strange fire before the Lord in the wilderness of Sinai'. For the details of this solemn incident, see Leviticus 10:1-7. Their sin was a manifold one, as Ellicott points out: (a) they each took his own censer and not the sacred utensil of the sanctuary; (b) they both offered it together whereas incense was only to be offered by one; (c) they presumptuously encroached upon the functions of the high priest, for according to the law the high priest alone burnt incense in a censer (cf Leviticus 16:12,13; Numbers 17:11); (d) they offered the incense at an unauthorised time, since it was apart from the morning and evening sacrifice.
All this is true: but the operative phrase in the incident is 'strange fire' ('strange' here has the force of 'unlawful' or 'common'). The presumptuous priests committed sacrilege by filling their vessels with common fire instead of taking it from the holy fire of the altar, which was always to be used in burning of incense (cf Leviticus 16:12,13).