In short, it’s a unit that powers other electronics using a rechargeable battery. They can be big enough for home backup or small enough to take camping. They generally have some combination of AC (wall) outlets, USB outlets, and sometimes less common types of outlets. What makes them different from other energy storage devices is that they balance high capacity with portability. https://www.lithiumbatterychina.com/lithium-ion-electric-bike-battery/more