IntelInside, IdiotOutside
jokes.
I think when they say this they mean that you can make harmonics of the same frequencies that are integer multiples of a fundamental frequency.
You know with a guitar, you know how you can make the same pitch (sound) with different strings by going down the bar lower and lower?? I think that is essentially the idea.
Musical instruments have to make different frequencies though, or else the sounds will sound all the same.