In this first step the camera has been placed on a tripod, and the shutter is open. One bolt of lightning strikes and is recorded on the image.
The shutter remains open to capture more lightning bolts. And sure enough, another bolt of lightning strikes and is recorded on the image.
The shutter stays open, and ... another bolt of lightning hits and gets recorded.
The shutter is still open and another bolt of lightning strikes. Finally, after a few strikes, the shutter is closed.