47 So Aaron took it as Moses said and ran into the midst of the assembly. And behold, the plague had already begun among the people. And he put on the incense and made atonement for the people.
48 And he stood between the dead and the living, and the plague was stopped.
49 Now those who died in the plague were 14,700, besides those who died in the affair of Korah.
Korah, an Israelite leader, has decided to stand up and oppose Moses and Aaron. He is presumably quite sick of having someone tell him what to do, and of being continually told off and suffering punishment for his unbelief.
Instead of successfully overthrowing the leadership that God has set, Korah and some of his followers experience the wrath of God with the ground opening up and swallowing them, and fire from God destroying others. God will always look after his own people, those who trust in him faithfully.
The next day the people are grumbling about the death of so many "people of the Lord" which leads to God sending a plague among them. The above verses detail the Old Testament priestly function of intercession once again - the priests are to be a 'go-between' for God and man to communicate God's commands and to make the people holy. Aaron makes the people holy symbolically through the incense and stops the plague in it's tracks.