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;

vertices
(
    ( 10 -60 530)
    (210 -60 530)
    (210  90 530)
    ( 10  90 530)
    ( 10 -60 730)
    (210 -60 730)
    (210  90 730)
    ( 10  90 730)
);

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

Run blockMesh:

 blockMesh
You can view its outcome in paraview:

 paraFoam