I think he is referring to the double-slit experiment in quantum physics?
If so, we really don't know why light behaves like that when it goes through the slits. If we put a detecting device, it suddenly, magically, behaves once again like normal light. Once the detecting device is gone (or turned off), the light again behaves queerly.
However, that does not stop us from theorising. Some people say light can also travel in waves sometimes. This property of light is also known as the wave-particle duality. So in certain cases, like waves, light interferes with itself. That is why in the double slit experiment, lots of bars are formed.
I am not a great expert in quantum physics, but I am sure there are a lot of people here who can tell you other theories, or expand on the one I have briefly explained above.