During your heart’s electrical conduction there are different phases. You have the contraction of the atria. The little pumps above the ventricle (main pumps) whose role is to help fill up the ventricles. The contraction of the ventricles. The main pumping part of your heart that pumps the blood to both your body and your lungs. And the repolarisation of both the atria and the ventricles. Basically the heart muscle reseting for another beat.

The atria contract first and repolarise first. On an ECG (heart trace) the atrial contraction corresponds to the P-wave. The little wave at the front. The atrial repolarisation occurs during ventricular contraction so you can see it on the ecg. When the ventricular contraction occurs you see the sharp up and down squiggles on the ECG. This is the QRS complex. Following ventricular contraction there is ventricular repolarisation. This occurs after the QRS complex and looks like a really big P-wave. This is the T-wave.

QTc prolongation is when the T wave takes a long time to occur after the QRS complex. When it gets too bad it is dangerous because the electrical energy that occurs in the T wave can set off another contraction of the heart. This contraction occurs outside of the normal pattern of the heart and the heart does not pump effectively leading to something called torsades de pointe which can develop into ventricular fibrillation leading to cardiac arrest (your heart not really pumping any blood).

Here is a labeled picture of an ECG ignore the u-wave its not really relevant and does not always occur.

Here is a picture of the heart showing the location of the atria and ventricular Heart