Pharaoh is the official title borne by the Egyptian kings down to the time when that country was conquered by the Greeks.
There are three Pharaohs who had historical significance in the Bible. The Pharaoh who was on the throne when Abraham went into Egypt was probably one of the Hyksos, or "shepherd kings". The Pharaoh on thr throne when Joseph was sold into slavery was probably Apopi, or Apopis, the last of the Hyksos kings. And the Pharaoh during the time of Moses, most famous of them all, was probably Ramesis II, though cases have been made for Amenhotep II, Merneptah, and Thothmes III.
For about seventy years the Hebrews in Egypt were under the powerful protection of Joseph. After his death their condition was probably very slowly and gradually changed. The invaders, the Hyksos, who for some five centuries had been masters of Egypt, were driven out, and the old dynasty restored. The Israelites now began to be looked down upon. They began to be afflicted and tyrannized over. In process of time a change appears to have taken place in the government of Egypt. A new dynasty, the Nineteenth, as it is called, came into power under Seti I, who was its founder. He associated with him in his government his son, Rameses II, when he was yet young, probably ten or twelve years of age. (Easton's Bible Dictionary)
During his forty years' residence at the court of Egypt, Moses must have known this ruler well - indeed grown up with him - and a considerable animosity must have developed between the two because of Pharaoh's obdurate refusal to give the Israelite's the freedom they demanded.
Because of the Pharaoh's hardheartedness, ten plagues were inflicted on the Egyptians before he let the people go.