The bounding box is a part of a plot that shows the limits of the XYZ data ranges. It is a good way to show the ranges when the settings produce a graphic that does not cover the entire range, as for some isosurfaces. It also provides a good reference system to orient the graphic output.

The isosurface on the left is difficult to interpret without any reference points.
The same isosurface on the right makes a lot more sense 
with the addition of a bounding box.