Easy:
Fibrillation is the concept of the various nodes of the heart working in sync to contract the ventricles and atrium of the heart. If the heart is erratic in how the atrium and ventricles contract, then you are in a state where the heart ventricles and atria are not in Sync. This irregularity is called fibrillation. Therefore you need to shock the heart to stimulate the SA and AV nodes (also the purkinje fibers) to restart, begin heart contraction again, and resume blood flow. When you defribillate, all you are doing is resetting the heart in hopes it will contract again.
