Who was Pharaoh? Why didn’t Pharaoh let the people go?
Here's the answer:
God hardened Pharaoh’s heart to punish Him and demonstrate God’s power.
Pharaoh was a title, like "President" or "King," and he was the leader of Egypt. During the time when the Jewish people were slaves in Egypt, the Pharaoh was mean and mistreated the Jews. When Moses asked Pharaoh if the Jews could go into the wilderness to worship God, Pharaoh said no. He wanted the Jewish people to keep working hard as slaves.
Moses went back again and continued asking Pharaoh to let the people go. When Pharaoh said no, God would send a plague to persuade him. Pharaoh would sometimes agree to let the people go so that God would remove the plague, but then he would change his mind. Then God would send another plague. After the tenth plague, Pharaoh obeyed and let God’s people go.
The Bible says that God hardened Pharaoh’s heart in order to demonstrate His power.
"So the magicians said to Pharaoh, 'God’s powerful finger has done this.' But Pharaoh remained stubborn. He wouldn’t listen, just as the LORD had said" (Exodus 8:19).
"Then the LORD spoke to Moses. He said, 'Go to Pharaoh. Tell him, "The LORD, the God of the Hebrews, says, 'Let my people go. Then they will be able to worship me. Do not refuse to let them go. Do not keep holding them back. If you refuse, my powerful hand will bring a terrible plague on you. I will strike your livestock in the fields. I will strike your horses, donkeys, camels, cattle, sheep and goats. But I will treat Israel’s livestock differently from yours. No animal that belongs to the people of Israel will die'"'" (Exodus 9:1-4).