God tells Moses that He will bring one final plague. All the firstborn of the land will die. There is no more discussion about Pharaoh letting the Hebrews go. The time for negotiation is over, and now God will let both the Hebrews and the Egyptians recognize that He is God.
The Hebrews will be saved from this plague by taking a lamb, bonding with it for five days, and then sacrificing the lamb as a substitute sacrifice from their firstborn. This is then instituted as an eternal remembrance in the month of Aviv (later known as Nisan). Leavening agents are to be purged at this time, and the idea of leavening as corruption is a common metaphor, which can be seen in 1 Cor 5:6-8.
Those who wish to partake of the Passover tradition are to be circumcised before they can add join in the tradition. This parallels baptism. The firstborn are to be redeemed, and this tradition is cited in Luke 2:23 when Jesus is taken to the temple.
The firstborn of the Egyptians die and the Hebrews are not only allowed to go, but are driven out.