Why is it called passover?
God unleashed 10 plagues on the Egyptians because they refused to obey Him and free their Hebrew slaves. The last plague killed the 1st born males in Egypt. Jews put blood from a sacrificed lamb on their doorposts so the angels knew to "passover" those homes and spare their sons from the plague.
The Jewish observance of Pesach, or passover, commemorates the final plague of Egypt. According to the Book of Exodus, the angel of Death struck down the first born of all of Egypt, but passed over and spared the children of Israel. After that, Pharaoh granted them their freedom.