What does 1 Kings 13:2 mean?

He shouted out against the altar, 'Altar, altar! This is what the Lord says: A son named Josiah will be born to David’s family. He’ll sacrifice the priests who serve here on you, and people’s bones will be burned on you.' 1 Kings 13:2 - Modern Text Bible

(The prophet predicts that a future king from David’s line will destroy this false altar and its priests.)