i think hyper did a great job in explaining it - doesn't get much better than that!
my two cents - the dumbed down version...
resting cardiac cell = polarized
when your heart is depolarizing (changing from just sitting there to contracting) you can think of it as contracting or systole. the contracting is not what is seen on the ecg. the ecg reflects the electrical impulses traveling through the heart. what makes it depolarize is the sudden rush of na+ from the outside of the cardiac cell to the inside where the cell concentration is higher in k+. as na+ rushes into the cell k+ goes outside. the ion concentration gets to a point that triggers the cardiac cell to contract or depolarize.
after depolarization, the cardiac muscle cell has to return to its original state again in order to recontract or depolarize again and again beat after beat. this process of returning to the original state ready to contract again is called repolarization.
repolarization (dystolye) can be broken down into two phases: absolute refractory and relative refractory.
as the muscle is repolarizing there is a period of time that if you were to introduce an electrical stimulus (like a shock, and think of a shock as a start button and the start button makes the heart muscle cell contract) it wouldn't be ready to contract. that phase of not being ready to contract again if stimulated by the shock is called the absolute refractory period (absolutely not am i going to contract) and the second half of repolarization is called the relative refractory period. this is where if you had a shock introduced (electrical impulse) it would fire, contract, start - however you think of contracting, and this isn't good because it hasn't fully recharged, reset. if the stimulus did arrive at just the right moment while the heart cell was in this particular phase of the cycle it could as hyper said cause deadly ventricular rhythms. that's why is important to have pacemakers interrogated and working properly because if you have failure to sense (just fires randomly without rhythm) it could fire during the relative refractory period and send the heart into bad rhythms.
i hope i was able to put it into some sort of understandable working perspective for you.