Declare good. Prophets of God receive their messages from God, not from men. It is the Lord who directs them and tells them what to say, whether that be in accord with the voices of others or not. The messenger who was dispatched to fetch Micaiah had a low idea of prophets in general when he thought that by such counsel as he was offering he could influence the message delivered.
That which is good. The good is not always that which appears to be good, or that which men may desire to hear. To encourage Ahab to go on this disastrous mission that would bring death to the king was not good from Ahab’s own point of view. Far better is unpalatable truth than welcome untruth.