What does Exodus 8:28 mean?

This is Pharaoh's counteroffer during negotiations to end the plague of flies (Exodus 8:25–27). God sent these catastrophes to convince the Egyptian king to release the nation of Israel from slavery (Exodus 1:11–14; 5:1–4). So far, Pharaoh has resisted. He certainly realizes that once the entire nation leaves Egyptian territory, those slaves aren't coming back. Yet the brutal infestation of flies (Exodus 8:24) has made him willing to talk. Moses has already rejected holding sacrifices in Goshen (Genesis 45:10; 46:34). But Pharaoh doesn't want the people moving too far away.

This is the second time Pharaoh has asked Moses to plead to the Lord God to stop a plague. The first time was in response to the plague of frogs (Exodus 8:8). He will later ask Moses to pray on his behalf for two more of the ten total plagues (Exodus 9:28; 10:17). Pharaoh never seems to attempt to speak to the Lord on his own; he only asks Moses to do so on his behalf as a messenger of the Lord. None of that changes Pharaoh's obstinate heart. In the end, he will always respond to mercy with more disobedience (Exodus 8:1–15, 31–32; 9:34; 14:5–7).