Background mesh is basic (initial) mesh to be refined. All the surface model must be placed inside the background mesh. It is created using blockMesh utility. The background mesh is simple block a bit larger than bounding box of the surface model. Example of blockMeshDict file:

convertToMeters 1;

vertices
(
    (-90 -100 310)
    (110 -100 310)
    (110  100 310)
    (-90  100 310)
    (-90 -100 510)
    (110 -100 510)
    (110  100 510)
    (-90  100 510)
);

blocks
(
    hex (0 1 2 3 4 5 6 7) (40 40 40) simpleGrading (1 1 1)
);

edges
(
);

boundary
(
    bkg_mesh_walls
    {
        type patch;
        faces
        (
            (0 3 2 1)
            (4 5 6 7)
            (0 4 7 3)
            (2 6 5 1)
            (1 5 4 0)
            (3 7 6 2)
        );
    }
);

Run blockMesh:

 blockMesh

Background mesh is very simple initial mesh used as an input for snappyHexMesh utility. You can view it in paraview:

 paraFoam