The diastole is the period during which the ventricles relax and gets filled again with blood for the next systole.
As with the systole, we can divide this period into three phases:
a. the rapid filling phase
b. the passive filling phase
c. the active filling phase.
Remember that during the systole, the AV-valves are closed. Blood that has meanwhile flown from the veins (VCS = vena cava superior, VCI = vena cava inferior and the pulmonary veins) into the atria have accumulated in that space.
At the beginning of the diastole, when the AV-valves open, this accumulated blood will suddenly and rapidly flow into the ventricles; this the rapid filling phase.
Once this accumulated blood has flown into the ventricles, blood keeps streaming in from the veins into the atria. This blood can immediately flow further into the ventricles: passive filling phase.
At the end of the ventricular diastole, the sinus node will become excited again and the atrial systole will start (remember that the atrial systole starts before the ventricular systole). This atrial excitation will induce atrial contraction and this will squeeze the last remaining blood into the ventricles; active filling phase.
As I said before, the diastole begins when the AV-valves open and ends when the AV-valves close (at the next systole)
The pressure in the ventricles during diastole is very low, close to zero mmHg. This is important as it must be lower than the low pressures in the atria, or else, blood would not flow into the ventricles.
Therefore, as shown in the diagram, there is not much change in the ventricular pressure during diastole, in contrast to the big change during systole.