An automated external defibrillator (AED), often called a "defib" for short, is a portable device used to help someone who's experiencing a sudden and life-threatening conditon called cardiac arrest. If a person suddenly collapses and stops breathing, they may be suffering from sudden cardiac arrest (SCA), which is very serious.
When someone has a cardiac arrest, their heart stops beating properly, which can be deadly if not treated quickly. A defibrillator works by delivering an electrical shock to the heart. This shock helps the heart to reset and hopefully start beating normally again.
Think of it like a jumpstart for the heart. The electrical shock from the defibrillator can help restart the heart's natural rhythm and save seomeone's life in an emergency situation.