In common useage, "void volume" is used as a synonym for the term "dead volume": the volume of liquid contained in the HPLC column. It represents (approximately) the minimum amount of solvent which must be pumped through the system before any injected sample can get to the detector (I said "approximately because this can actually get to be complex if you get into more advanced discussions). It does not affect resolution directly (i.e., it is not used in calculating resolution).
It cannot be calculated exactly, but it can be approximated if you know the % pore volume of your packing (about 35% for many common "reversed phase" packings). It is more commonly measured, either approximately, by noting the time delay for the injection disturbance to perturb the baseline, or exactly by injecting a non-retained but detectable solute.
Sometimes, "void volume" is used to describe a cavity at the head of the column or unswept volume in tubing or fittings. Used in this context, void volume will decrease resolution (by providing place for the peaks to broaden). It is not, however, incorporated directly into the calculation.
Hope this helps.
