What does Genesis 46:12 mean?

Judah’s sons were Er, Onan, Shelah, Perez, and Zerah (but Er and Onan died in Canaan). Perez’s sons were Hezron and Hamul. Genesis 46:12 - Modern Text Bible

(Judah’s sons are listed, with a note that two died earlier. Perez’s sons are also named.)