What does Leviticus 10:1 mean?

Aaron's sons, Nadab and Abihu, each took their own fire pan, put fire and incense in them, and offered it to the Lord in a way He hadn't commanded. Leviticus 10:1 - Modern Text Bible

(Nadab and Abihu act on their own, bringing an offering that God didn't ask for. This is a serious mistake, showing disrespect for God's instructions.)