Background mesh is a very simple initial mesh. It serves as an input for snappyHexMesh utility. It is a basic (initial) mesh to be refined. All the surface model must be placed inside the background mesh. Usually, background mesh is a simple block a bit larger than bounding box of the surface model. Alternatively, the background mesh can be used to create inlet or outlet of the final mesh (this case).

It is created using blockMesh utility.

Example of a blockMeshDict file:

convertToMeters 1;

vertices
(
    (-90 -100 200)
    ( 80 -100 200)
    ( 80  100 200)
    (-90  100 200)
    (-90 -100 300)
    ( 80 -100 300)
    ( 80  100 300)
    (-90  100 300)
);

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

You can view the outcome in paraview:

 paraFoam