https://www.youtube.com/watch?v=NSb8jmfPuYk
the clapping is audience members signaling to one another they thought the movie was pretty good. if your claps are joined by other claps, there's a vague, fleeting sense of a having shared an enjoyable evening out with like-minded friends – an increasingly rare occurrence. they don't think the movie screen can hear the clapping, you eunuched faglet.