What does Exodus 6:21 mean?

Izhar (Exodus 6:18) was the second son of Kohath the son of Levi (Exodus 6:16). Izhar had three sons named Korah, Nepheg, and Zichri. Nepheg and Zichri are not mentioned again. Korah was a leader of the rebellion in the wilderness (Jude 1:11), fully described in Numbers 16. According to this genealogy, Korah and Moses were cousins. Korah would rebel against both God and his own family member. Izhar would become leader of the Izharites or Izeharites (Numbers 3:27; 1 Chronicles 24:22; 26:23, 29).

Kohath had four sons (Exodus 6:18), yet the descendants of only three are described in this verse and the surrounding context (Exodus 6:20, 22). Hebron is omitted, though Hebronites are mentioned in Numbers 3:27. They may have had no additional family members to mention. The following verse mentions Mishael and Elzaphan (Exodus 6:22), later noted for their role concerning the bodies of Nadab and Abihu (Leviticus 10:4). Though not always readily apparent to modern readers on the first read, each group of descendants noted in this list includes later connections important in the story told by the Bible's first five books.