A Power bank or Battery pack is an external battery that you can use to power your dashcam. The Power bank connects directly or with a cigarette power supply to the battery of your car and the dashcam connects back to the Power bank. So the Power bank is actually between the power supply of the car and the dashcam.
While driving, the Power bank is simultaneously charged and the dashcam is powered. When the ignition of the car is turned off the capacity of the Power bank is used to power the dashcam. In this way, you can also use the dashcam as security while parking without fear of being left with a dead battery.