Albert Barnes Commentary Leviticus 10:1

"And Nadab 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 Jehovah, which he had not commanded them." — Leviticus 10:1 (ASV)

Nadab and Abihu - The two elder sons of Aaron (Exodus 6:23; Numbers 3:2), who were among those invited to accompany Moses when he was going up Mount Sinai, but who were to worship afar off, and not come near the Lord. (Exodus 24:1–2).

Censer - See (Exodus 25:38 note).

Strange fire - The point of their offence is evidently expressed in this term. This may very probably mean that the incense was lighted at an unauthorized time. And we may reasonably unite with this the supposition that they were intoxicated , as well as another conjecture, that they made their offering of incense an accompaniment to the exultation of the people on the manifestation of the glory of the Lord (Leviticus 9:24).

Since they did not perish within the tabernacle but in front of it, it seems likely that they may have been making an ostentatious and irreverent display of their ministration to accompany the shouts of the people on their way toward the tabernacle.

The offence for which they were immediately visited with outward punishment was thus a flagrant outrage on the solemn order of the divine service, while the cause of their offence may have been their guilty excess.