This passage in Leviticus tells of the drastic difference between Aaron the High Priest, and his two eldest sons, Nadab and Abihu. Aaron's sacrifices were accepted by God whereas his two sons who kindle an 'unauthorised fire' that was not commanded by God, were immediately consumed by fire sent from God. This event shows forth clearly an often forgotten truth of who God is: that He is totally holy, and infinitely glorious. Yet in the Scriptures, God's holiness is lifted up as one of his most important attribute.
It is so critical we properly see God's holiness, because only then will we ask the most important question the bible ever asks: "how could unworthy and sinful human beings find acceptance from the infinitely holy and worthy Lord of the universe"? Have you personally found the answer of God's word to this question? There is nothing more important in one's life!