It’s something to do with blocking. Imagine the sun shining at you. If something moves to block you, the sun will not be shining at you, correct?

Same idea. At the point where the ball passes the hole, there is no light detected since the ball is the one receiving the light.

At first, the intensity is more than zero. When the ball passes through, the graph goes down to zero. Once the ball passes through and there is no blockade, the graph goes back up again. This process repeats itself again.

There are four occurrences of the graph going down to zero, hence the ball passes four times.

Some horizontal lines at zero are longer than the rest. This shows that the balls travel at different speeds. Longer lines come about because the ball passes the hole more slowly (and hence, more time is needed to pass the hole, explaining the longer length).

isn't that the same question being asked as Tishar?