Chapter 9: Atonement and Sacrifice
Aaron makes atonement by sacrifice for himself and all of Israel. The glory of the Lord appears unto all of Israel and the Lord consumes the offering on the altar by fire.
Chapter 10: Unauthorized Sacrifices
Nadab and Abihu, the sons of Aaron perform un-authorized sacrifices, and as a consequence are slain by a fire from the Lord. Aaron and his other sons are forbidden to mourn for their loss, because of the iniquity that they did. The Lord forbids Aaron and his brethren to drink wine or strong drink, being the difference between clean and unclean and holy and unholy (Leviticus 10:9-10).
This is interesting, because God is telling Aaron not to drink alcohol, and in the LDS church today, we are commanded to not drink as well. It is because the Lord has commanded us not to drink alcohol.
We also learn from this chapter that, you need the authority from God to perform ordinances. The Lord killing Nadab and Abihu for performing these ordinances without the proper authority proves how important it is to the Lord, that we have the correct and proper authority to perform ordinances.