Background designers design the scenery around, or on top of, where the animation happens. Once the storyboard has been signed off, they use the storyboard panels as a basis and inspiration from which to create detailed backgrounds for each scene.
Sometimes multiple scenes will use the same background, so, firstly, they need to make a list of all the backgrounds that need to be designed for the film. They create designs which fit the style of the animation, thinking about colour, texture and light. Background designers also watch out for the way objects in the foreground and background of scenes will interact and make sure that everything lines up appropriately and that nothing is too distracting.