Here is a short video clip showing how an image can be built up on a TV screen, one line at a time. In a real TV set, the beam moves much more quickly (about 7000 m/s!) across the screen and 25 complete images are shown each second.
raster scan from mr mackenzie on Vimeo.
Remember that in a real TV set, the beam does not scan its way back up the screen but takes a “flyback” from the bottom right to the top left corner to start on the next image.