Hi basha009,
This means exactly what it says - if this option is checked, the fields inside the volumetric object will be solved.
You generally want to have this option checked for all materials except for metals. This is because solving for the decaying fields inside of metals will require an extremely fine mesh, and your resulting simulation will have a huge mesh and take forever to solve.
If you leave this value unchecked, HFSS simply applies a boundary condition to the surface of the object and doesn't solve inside of it.