What does 1 Chronicles 24:19 mean?

These were the assignments for the priests, organized by their families, for when they would come to serve in the temple, just as the Lord had told their ancestor Aaron. Everything was done exactly as God had commanded him. 1 Chronicles 24:19 - Modern Text Bible

(This verse sums up the system: each family had a set time to serve, following the instructions God gave to Aaron long ago.)