What does 1 Chronicles 24:1 mean?

Aaron's sons were Nadab, Abihu, Eleazar, and Ithamar. Nadab and Abihu died before their father and had no children, so only Eleazar and Ithamar served as priests. 1 Chronicles 24:1 - Modern Text Bible

(This verse explains why only two of Aaron's sons continued the priestly line.)