This is a wonderful insight into what goes on inside a DSLR when the shutter fires. It shows the mirror moving out of the way of the light coming in through the lens, so that the light can hit the sensor.
And it shows the shutter opening and closing, letting the light onto the sensor for just the right amount of time.
And (yes, there’s more!) it shows how the shutter works for very fast shutter speeds, when it doesn’t open the whole way, but travels across the frame in a slit, like a torch/flashlight illuminating a scene by scanning across it.