What does 1 Kings 1:25 mean?

'Because today Adonijah has gone and sacrificed a lot of cattle, fattened calves, and sheep. He invited all the king's sons, the army commanders, and Abiathar the priest. They're eating and drinking with him and shouting, “Long live King Adonijah!”' 1 Kings 1:25 - Modern Text Bible

(Nathan describes the scene at Adonijah's feast, showing that Adonijah is acting as if he's already king.)