What is an AED?

An automated external defibrillator (AED) is a lightweight, portable device that delivers an
electric shock through the chest to the heart. The shock can potentially stop an irregular heart beat (arrhythmia) and allow a normal rhythm to resume following sudden cardiac arrest (SCA).
SCA occurs when the heart malfunctions and stops beating unexpectedly. If not treated within minutes, it quickly leads to death. Most SCAs result from ventricular fibrillation (VF).
VF is a rapid and unsynchronized heart rhythm that originates in the heart’s lower chambers
(the ventricles). The heart must be “defibrillated” quickly, because a victim’s chance of surviving drops by seven to 10 percent for every minute a normal heartbeat isn’t restored.

Source: https://www.heart.org/-/media/data-import/downloadables/pe-abh-what-is-an-automated-external-defibrillator-ucm_300340.pdf