Nadab is the son of Aaron, and one of the first Jewish priests. He explains the system of the laws in the book of Leviticus and how important it is to follow them exactly. He should know. He didn't and God killed him.
Moses is nearing the end of his life. The Israelites are ready to cross the Jordan River and invade Canaan, but he will not be allowed to join them. As they wait on the plain of Moab, Moses reminds them of all that has happened over the last 40 years.