Background mesh is a basic (initial) mesh to be modified. All the surface model must be placed inside the background mesh. It is created using blockMesh utility. The 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).

Example of blockMeshDict file:

convertToMeters 1;

x1      -85;
x2       76;
y1      -91;
y2       75;
z1      203;
z2      282;

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) (28 28 15) simpleGrading (1 1 1)
);

Run blockMesh:

 blockMesh

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

 paraFoam