What does 1 Kings 1:11 mean?

Nathan went to Bathsheba, Solomon's mother, and said, 'Haven't you heard that Adonijah, Haggith's son, has made himself king, and David doesn't even know about it?' 1 Kings 1:11 - Modern Text Bible

(Nathan, the prophet, alerts Bathsheba to the crisis. David is unaware, and Adonijah is moving fast to secure power.)