Original: They begin their journey together to stop Darken Rahl and prevent him from opening the boxes of Orden: magical devices which can give absolute power over life and death. Kahlan tells them that Rahl has two of the boxes but requires the third before he is able to make the magic work.
Simplified: Zedd then decided that they need to cross the boundary to stop Darken Rahl from conquer the world using the magic of the boxes of Orden.