Okay, I'd better try to be a little less flippant this time.
As I understand it (and, as always, I am willing to be corrected)passive noise reduction is merely the fact that you've got a great big padded headset between your delicate little ears and the big old noisy machine.
Active noise reduction means that the clever old headset listens to the noise that's managed to get in and then creates more noise but this is, if you like, inverted noise so when you add together the original noise and the new inverted noise you actually get less noise.
That's me, trying not to be flippant. I'm sure others can add to that explanation but, as anyone who ever tried to teach me physics will testify, that's about as far as my brain goes.
Bad Noise + Good Noise = Less Noise