This verse is part of the narrative about the enemies God stirred up against Solomon as a punishment for his idolatry. The verse specifically refers to Hadad the Edomite. Here's a breakdown of its meaning:

"When Hadad heard in Egypt that David slept with his fathers..." This means Hadad, who was in exile in Egypt, learned that King David of Israel had died (a euphemism for "died naturally" in biblical language).

"...and that Joab the captain of the army was dead..." Hadad also learned that Joab, David's powerful and often ruthless military commander, had died. Joab had likely been a major obstacle to any attempt by Hadad to return to Edom and reclaim power.

"Hadad said to Pharaoh, 'Let me depart, that I may go to my own country.'" Upon hearing this news, Hadad saw an opportunity. With David and Joab gone, the political landscape in Israel and its surrounding territories had shifted. He asked Pharaoh, who had been harboring him, for permission to leave Egypt and return to his homeland of Edom. The clear implication is that Hadad intended to try and re-establish his own rule in Edom and potentially pose a threat to Solomon's kingdom.

In essence, this verse signifies a change in circumstances that allowed Hadad to see an opportunity to return to his homeland and potentially challenge Israel's power. It's a setup for future conflict and serves as a sign of God's judgment on Solomon's actions.

The verse highlights the following:

Political Instability: The death of a strong ruler like David often created instability and opportunities for others to challenge the status quo.
Strategic Timing: Hadad's decision to leave Egypt was calculated. He waited until the most opportune moment to act.
Divine Punishment: The story of Hadad (and other adversaries) is presented as part of God's punishment of Solomon for his unfaithfulness.
Foreshadowing: The verse foreshadows potential conflicts and challenges to Solomon's kingdom that will arise due to his disobedience.