An ignorant question about impedance

I've read over these many years about the importance of properly matching a preamplifier's output impedance with an amplifier's input impedance. I've always just accepted this as something that has got to be dealt with by folks who understand such things. But now I'm wondering: What happens when there is a mismatch? Is there some threat to the equipment, or does the reproduction of music suffer in some obviously discernable fashion? (Highs or lows or mids are like this or that or some such?) I recognize that this is a boneheaded query, but I've found myself wondering and I've not been able to find the answer through my online searches. If someone could point me in the right direction, my non-technical mind would rest easier. Thanks.