The first ten chapters of Numbers deals with:
The numbering of the people at Sinai, and preparations for resuming their march (1–10:10).
The main characters in the Book of Numbers in the Old Testament of the Bible are Moses, Aaron and his sons, and the Levites. Because God had taken the lives of the oldest son in each Egyptian family as the Israelites left Egypt, God claimed the oldest son of each Israelite family as His own. However, as the laws were given by God to Moses, two of Aaron's sons died.
At the time when the Lord spoke to Moses on Mount Sinai, Aaron's sons were: Nadab (his oldest), Abihu, Eleazar, Ithamar. All were anointed as priests and set apart to minister at the Tabernacle. But Nadab and Abihu died before the Lord in the wilderness of Sinai when they used unholy fire. And since they had no children, this left only Eleazar and Ithamar to assist their father Aaron. (Numbers 3:1-4)
The conflict or difficulty, then, is that Aaron and his sons, who are the priests of the Tabernacle, have lost two of their number and cannot fulfill their tasks with so few of them to work. This is when God claims the Levites as His own.
It is important to note that...
According to the third chapter of the Book of Numbers...
Then the Lord said to Moses, "Summon the tribe of Levi and present them to Aaron as his assistants. They will follow his instructions and perform the sacred duties at the Tabernacle on behalf of all the people of Israel. For they are assigned to him as representatives of all the people of Israel. They are in charge of all the furnishings and maintenance of the Tabernacle. However, only Aaron and his sons may carry out the duties of the priesthood; anyone else who presumes to assume this office shall be executed."
And the Lord said to Moses, "I have accepted the Levites in substitution for all the oldest sons of the people of Israel. The Levites are mine in exchange for all the oldest sons. From the day I killed all the oldest sons of the Egyptians, I took for myself all the firstborn in Israel of both men and animals! They are mine; I am Jehovah." (3:5-13)
In essence, Numbers is the book of the directions—laws. The censuses count the members of the tribes of Israel. Rules and jobs are established. E.g., two silver trumpets are made and the manner of their sounding indicates when the people are to gather, when only the leaders are to gather, and which tribes will move first when it is time for the Israelites to move on.
There is a Cloud over the Israelites. They are to move when the Cloud moves and stay in place as long as the Cloud does not move. They may linger for weeks or for a year—only moving with the Cloud.
A rule or law is given as to how a man should conduct himself when accusing his wife of adultery, what the priest must do, and what the wife must do. There are consequences in the event that the woman is guilty.
Special arrangements are even made for the Levites because God has chosen them especially to serve him by helping Aaron and his sons—they have specific jobs—even the moving of all the pieces of the Tabernacle when the tribes of the Israelites move.
All of these things are put in place to govern the tribes as they travel to the Promised Land. Moses relays God's commands; Aaron and his family are the only priests of the Tabernacle. The Levites' responsibilities are to serve God by helping care for the Tabernacle. (Numbers 1-10)