I have given the Levites as a gift to Aaron and his sons from among the Israelites, to do the work for the Israelites in the tent of meeting, and to make atonement for the Israelites, so there will be no plague among the Israelites when the Israelites come near the sanctuary.” ¹ 

Then Moses said to Aaron, “Take the censer, put burning coals from the altar in it, place incense on it, and go quickly into the assembly and make atonement for them, for wrath has gone out from the Lord – the plague has begun!”

So Aaron did ¹  as Moses commanded ²  and ran into the middle of the assembly, where the plague was just beginning among the people. So he placed incense on the coals and made atonement for the people.