A friend one took a timed exposure photo of me which lasted only a few seconds, but I had to remain perfectly still, no blinking. It made for amazing sharpness. It was gorgeous.
To take a simple photo, light is exposed to the receptor. In usual cameras, it is film. That light carries the image. To do a very long exposure like that, they can map what is moving, from what is stationary. Also, stationary objects can be judged for distance. The slower something appears to move, the farther away it is. Take photos from the passenger window of a car, going 50 or faster. Stuff in the distance will be in several pictures, appearing much the same. Things at the road’s edge will be each in only one shot.
With shutters remaining open, nearby objects would leave long streaks. Far away objects would remain pin pricks. Objects so far away, very dim light reaches this far, would accumulate enough light in one spot, to be visible.