The book of Numbers is the story of two generations, as indicated by two censuses. Except for Joshua and Caleb, those Israelites who left Egypt as adults died during the forty years in the wilderness (Num 26:64-65). It was those who left Egypt as children or were born during the forty years who ended up reaching the Promised Land.
The census occurs "after the plague" (26:1) and is the next thing mentioned after the incident with the Moabite and Midianite women. The purpose of the census is to organize how the land of Canaan will be divided among the tribes of Israel. Here we see another example of God's mercy, as the plan for Israel to proceed to the Promised Land continues despite the propensity of the people to sin.
The results of the census are prefaced with the statement: "The people of Israel who came out of the land of Egypt were" (v 4). There is a tradition that each generation of Israelites should think of itself as having pesonally come out of Egypt, based on statements like this one. At this point, in year 40 of the Exodus, the adults who came out of Egypt have died. Those counted in the second census either left Egypt as children or were born in the wilderness. But in some sense they had all "come out of Egypt."
The section on the tribe of Reuben mentions the rebellion, years before, in which Reubenites Dathan and Abiram played a major role. Those particular lines of Reubenites would not be inheriting land, since they had been swallowed up by the ground (vv 9-10).
Verse 11 gives up the important information that the children of Korah the Levite, the leader of the rebellion, had not died. Descendants of Korah would include, most notably, the prophet Samuel. Some descendants of Samuel would become leaders in tabernacle and temple worship and would author a dozen psalms, psalms known for an emphasis on God's mercy and the hope of resurrection.
However the numbers in Num 26 should be interpreted, we see God's faithfulness in maintaining Israel's population at the same level during the wilderness years. There were 603 elephs in the Israelite army at the beginning of year 2 of the Exodus, and 601 in the second census in year 40.
The results of the census would be used, along with some kind of randomizing method, to apportion the land. The results of the process are recorded in the book of Joshua, which does not mention how the randomization worked. One Jewish tradition pictures lots that spoke the name of a tribe. This tradition is based on Num 26:56, which literally refers to the "mouth" of a lot.
In any case, the distribution was directed by God. That's the reason for casting lots, which is a request for God to make a decision. As we read in Proverbs 16:33, "The lot is cast into the lap, but its every decision is from the Lord." Such a process was used, for example, by the first Christians in choosing a replacement for Judas to reconstitute the Twelve (Acts 1).
