What does Exodus 8:25 mean?

Moses said, As soon as I leave you, I’ll pray to the Lord, and tomorrow the flies will leave you, your officials, and your people. But don’t lie to us again by refusing to let the people go and offer sacrifices to the Lord. Exodus 8:25 - Modern Text Bible

(Moses agrees to pray, but warns Pharaoh not to go back on his word this time.)