1 Now Nadab and Abihu, the sons of Aaron, each took his censer and put fire in it and laid incense on it and offered unauthorized fire before the LORD, which he had not commanded them.
2 And fire came out from before the LORD and consumed them, and they died before the LORD.
This is a dramatic account of two of Aaron's sons who did not take their role as priests seriously enough. That should be a warning to us to listen to the words that God has revealed to us (in the bible) and to act as God has commanded us.
Where Nadab and Abihu went wrong was that they tried to do something in a way that God hadn't commanded them to do it. Our lesson, more specifically, is to ensure that we follow God's example for worship. We need to look at the way in which Jesus and his Apostles taught the early Christians to worship and we should try to take the timeless principles from that.
Because God would not even spare the sons of Aaron, who was very close to God and favoured by him, the other priests and Israelites would have received a very strong lesson on the holiness of God and the importance of serving him faithfully.