Background mesh is a basic (initial) mesh to be modified and/or refined. Entire surface model must be placed inside the background mesh. Background mesh is created using blockMesh utility. Background mesh is simple block a bit larger than bounding box of the surface model. Alternatively, background mesh can be used to create inlet or outlet of the final mesh (as in this case).

Example of blockMeshDict file:

convertToMeters 1;

x1      -75;
x2      105;
y1      -77;
y2      85;
z1      330;
z2      450;

vertices
(
    ($x2  $y1  $z1)
    ($x2  $y2  $z1)
    ($x1  $y2  $z1)
    ($x1  $y1  $z1)
    ($x2  $y1  $z2)
    ($x2  $y2  $z2)
    ($x1  $y2  $z2)
    ($x1  $y1  $z2)
);

blocks
(
    hex (0 1 2 3 4 5 6 7) (12 12 6) simpleGrading (1 1 1)
);

edges
(
);

boundary
(
    w1
    {
        type patch;
        faces
        (
            (1 2 6 5)
        );
    }

   w2
    {
        type patch;
        faces
        (
            (5 6 7 4)
        );
    }

   w3
    {
        type patch;
        faces
        (
            (4 7 3 0)
        );
    }

   w4
    {
        type patch;
        faces
        (
            (0 3 2 1)
        );
    }

   w5
    {
        type patch;
        faces
        (
            (0 1 5 4)
        );
    }

   w6
    {
        type patch;
        faces
        (
            (2 3 7 6)
        );
    }

);

Run blockMesh :

 blockMesh

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

 paraFoam