The setup of this video is very easy to explain. It features a guy with a fart machine walking around Times Square and pressing the button in order to catch out New York City tourists. Simples.
Some people react nonchalantly, barely giving the prankster a glance. Maybe they live with a particularly windy person and so are used to hearing someone blow off on a regular basis.
Others react a little more strongly, laughing, staring, or moving away from the offensive (non) smell at speed. They must be the tight-arsed people who never break wind themselves and so aren’t used to such a base act.
This prank isn’t big or clever, but it is funny. And being someone who is childish enough to own a fart machine of his own (and having bought two people them for Christmas) I take inspiration from this video.