Leviticus says Nadab and Abihu offered “unauthorized fire” before the LORD, which he had not commanded them. Fire came out from before the LORD and consumed them, and they died. Moses summarizes the event as showing that God will be regarded as holy among those who come near him (Leviticus 10:1–3).
In Leviticus 10:1–3, Aaron’s sons Nadab and Abihu perform an incense offering using “unauthorized” fire—something Yahweh had not instructed. Immediately, a fire comes out from Yahweh’s presence and consumes them, and they die there in front of Yahweh. This episode comes right as the priesthood begins active service after the tabernacle and priests have been prepared and installed (Leviticus 8–9). The previous scene ends with fire from Yahweh appearing as a sign of acceptance at the altar (see Leviticus 9:23–24).
Leviticus 10:1–3 portrays a failure by insiders at the very center of Israel’s worship.
A careful answer keeps the immediate passage, the book's argument, and the whole-Bible storyline together, so the question is answered in context instead of as a detached idea.
Start with Leviticus 10:1-3, then read the surrounding section so the answer stays connected to the passage's flow.
1Nadab and Abihu, the sons of Aaron, took each of them his censer, and put fire therein, and laid incense thereon, and offered strange fire before Yahweh, which he had not commanded them.
2There came forth fire from before Yahweh, and devoured them, and they died before Yahweh.
3Then Moses said to Aaron, This is it what Yahweh spoke, saying, I will be sanctified in those who come near me, and before all the people I will be glorified. Aaron held his peace.