The LORD said to Moses, "Yet one plague more I will bring upon Pharaoh and upon Egypt. Afterward he will let you go from here. When he lets you go, he will drive you away completely.
Pharaoh refused time after time to let Israel go. God warned him again and again, but when he didn't change his mind, God visited plague after plague on the land of Egypt. Turning the Nile to blood, and sending frogs, insects, sickness, and more. The people and the land were suffering terribly, yet Pharaoh continually hardened his heart and kept Israel captive.
Now God will send a final, terrible plague. The first-born of every household in Egypt will die if Pharaoh does not relent. This will be such a fearful event that Pharaoh will not only relent and allow them to go, but he will actually drive them away completely.
An important theme through the bible is how God causes people to believe in him. It is interesting here to see that such amazing works of power never led to Pharaoh having a saving faith in God. The same is true for us today - miracles will not cause someone hard of heart to believe. Faith comes more mysteriously, by hearing the Word of God.