The ten plagues are found in the book of Exodus. In this book, God sends these plagues against the king of Egypt (pharaoh), because the king will not allow the people to go into the wilderness to worship God. In light of this tension there is a contest between God and Pharaoh.
God sends plague after plague through Moses, until Pharaoh gives in and allows the Israelites leave. The significance of this is twofold. First, these plagues show that God is more powerful than any earthly king. Second, the plagues show that God is faithful to his people and he deliver them from bondage and slavery. This idea of the exodus will be one of the major imageries of the Bible, which signifies salvation.