Americans all have their own Super Bowl traditions, like the many unique unhealthy snacks various states prefer, but one that is shared all over the country is someone loudly explaining how that yellow first down line gets superimposed on the field. Traditionally, the specifics of the story are made up on the spot by the explainer, but popular themes include a claim that it's beamed down from space and often involve recently declassified military technology. The true explanation is both way simpler and at the same time more complex, because it's a real answer about how technology works. The helpful folks at Vox created this explainer video that will walk you through it in about four minutes, so you can cough and politely make everyone in the room realize that you are the most knowledgeable and deserve the last piece of the party sub:
Basically, the field is a big green screen, except it's also a huge painted field of grass with people running all over it and cameras whirring around willy-nilly. In order for the green screen to work, then, the three cameras in the stadium need to memorize what every inch of field (and player uniform) looks like in every possible condition. No matter what happens, no matter how fast the cameras are moving, the system (known as "1st and Ten") knows which pixels are field, where on the field they are, and which pixels are players, balls, refs, and other non-field things.