The placement of this instruction suggests that Nadab and Abihu might have been drinking alcohol when they offered the "unauthorized fire" (Leviticus 10:1). Priests should not drink anything that could impair their judgment before serving in the tabernacle.
the Lord said to Aaron: Previously, God communicated only with Moses, but now he accepted Aaron as the people's representative and spoke directly to him.
The word common means "secular" or "ordinary."
clean and the unclean: See Leviticus 7:19–21; 11:1–15:33.