I don't understand this. Delayed how? Does this mean if you are watching a video, and the person is talking, that a delay of milliseconds between the person's l

I don't understand this. Delayed how? Does this mean if you are watching a video, and the person is talking, that a delay of milliseconds between the person's lips moving and when you hear it? But what does that have to do with castinets? I wouldn't be able to see them they probably move so fast. But they don't mention video matched against audo delay, that I read. If you start a song and they delay it by 10 seconds, it just starts 10 seconds later, how would you even know there's a delay? Or if there's a delay in the middle of a song, how would you know any difference, as maybe that's how the music piece is supposed to sound. What am I not getting?