Ventricular fibrillation
This problem is called "ventricular" fibrillation because it affects the ventricles of the heart.  The ventricles are the biggest parts of the heart.  Their job is to pump blood (which carries oxygen) to the lungs or the rest of the body.  When a person is in ventricular fibrillation, the ventricles cannot do this job.  This means that the lungs, brain, and other organs will not get enough blood or oxygen.