The DVD drive uses a laser to shine on the surface of the DVD. A lens creates an upside-down, right-side-up image of this surface on the bottom (the tracking) stage.
The laser turns one pixel at a time into either light or dark. A sensor records whether or not there is light hitting it. If it detects that there is no light arriving within a certain period of time, it assumes that the pixel is black and writes code 100 to its “command” register. This command will tell the circuitry inside the DVD player’s digital video circuitry to turn off that pixel on-screen by telling it not to send light back for any length of time (we call this making a “black frame”).