What does Genesis 10:21 mean?

This verse begins the final section of chapter 10's table of nations, describing how all of the people groups of the ancient world descended from Noah's three sons. This section on the line of Shem comes last, though this verse makes clear that Shem was the oldest of Noah's sons. Earlier, Ham was described using a Hebrew term meaning either "youngest," or "least" (Genesis 9:24). Here, translations are split on whether Shem or Japheth is the older brother. Of course, their exact birth order is not explicitly given anywhere in the Bible. Most likely, but not assuredly, Shem was the oldest of the three sons Noah brought with him on the ark (Genesis 7:1). Shem's line is the one that will lead to Abraham and the Israelites.

Verse 21 begins by referring to Shem as the father of the Eberites. This is typical of genealogies, especially in the ancient world. The term "father" is often used to refer to any male ancestor (Genesis 15:15; Genesis 31:3). This makes Shem the "father" of the Eberites, in the sense that, as Eber's great grandfather, Shem is the ancestor of that people. Through the Eberites, eventually, Shem's line will lead to Abraham (Abram).