The cardiac cycle was once concept I struggled with during nursing school because it involves multiple aspects of cardiac function. A diagram like the one below is how I could make sense of both because it shows the cardiac conduction system along with corresponding waveforms.
The P-wave on an electrocardiogram (EKG) represents the depolarization of the atria- the spread of the electrical impulse from the SA node throughout both atria. The QRS complex represents the depolarization of the ventricles, which is when the electrical impulse spreads throughout the ventricles. During this time, the atria repolarize, which is not visible on the EKG tracing. The T-wave on an EKG represents repolarization of the ventricles.