Numbers 26 is a census of the Israelites taken after a plague and just before they are about to enter the Promised Land. Verse 49 is simply listing the families descended from specific ancestors.

Here's a breakdown of what the verse means:

"Of Jezer, the family of the Jezerites..." This means that a man named Jezer was the ancestor of a group of people. All those descended from Jezer were known collectively as the "Jezerites." They formed a distinct clan or family within the larger tribe.
"Of Shillem, the family of the Shillemites." Similar to the previous part, this indicates that a man named Shillem was the ancestor of the "Shillemites." They formed another distinct clan or family within the tribe.

In essence, the verse is:

Genealogical: It's recording the lineage and descendants of specific individuals.
Organizational: It's showing how the larger tribe was structured into smaller family units based on ancestry.
Census-related: It's part of a larger census that aimed to count the number of fighting men in each family, and therefore the strength of each group for potential land distribution and military service.

These family groups were important for maintaining social cohesion, land ownership, and identity within the Israelite community. The census in Numbers 26 ensured that each family was properly accounted for in preparation for entering the Promised Land.