What does 1 Chronicles 5:1 mean?

Reuben was Israel’s oldest son, but because he slept with his father’s concubine, his rights as the firstborn were given to Joseph’s sons. So Reuben isn’t listed as the firstborn in the records. 1 Chronicles 5:1 - Modern Text Bible

(This verse explains why Reuben lost his special status as the oldest son—his actions had consequences, and the privilege went to Joseph’s family instead.)