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.

It is created using blockMesh utility.

Example of blockMeshDict file:

convertToMeters 1e-3;

// Dx = 180 mm, dx = 10 mm
// Dy = 150 mm, dy = 10 mm
// Dz = 200 mm, dz = 10 mm
vertices
(
    (-165  -20   530)
    (  15  -20   530)
    (  15  130   530)
    (-165  130   530)
    (-165  -20   730)
    (  15  -20   730)
    (  15  130   730)
    (-165  130   730)
);

blocks
(
    hex (0 1 2 3 4 5 6 7) (18 15 20) simpleGrading (1 1 1)
);

Run blockMesh:

 blockMesh

You can view its outcome in paraview:

 paraFoam