K prime simply means by how many times of unretained peak (solvent peak) also called void time can the column retain the main peak (T-T0/T0)
while the void volume is the volume of mobile phase required to elute the unretained peak (solvent peak).
For the flow rate of 1 ml/min the retention time of the unretained peak (solvent peak) also called void time T0 is equal to the void volume