Aaron's sons Nadab and Abihu (Exodus 6:23): These were Aaron's oldest sons who had gone with Moses and the seventy elders to Mount Sinai (Exodus 24:1, 9). They also saw the great theophany (appearance of God) there. They died without male children (see also Exodus 28:1; Numbers 3:2–4; 26:60–61; 1 Chronicles 6:3; 24:1–2).
censers: These were fire pans or trays for holding hot coals. They sprinkled incense on them (Exodus 30:34–38; see also Leviticus 16:12–13; 2 Chronicles 26:19; Revelation 8:3–4).
unauthorized fire: The Hebrew phrase ’esh zarah means “strange fire.” This could mean “foreign,” and thus “unauthorized” (see Exodus 30:9) or even “pagan” (as in Psalm 44:20; 81:9).
It seems that Nadab and Abihu used fire from a source not approved by God (Numbers 3:4; 26:61). It could have been even a pagan source. They might have offered incense not prepared according to God's instructions (Exodus 30:34–38). The text does not mention that though. They entered God’s presence in their own way, not his.