The first thing that a car audio amp does is boost the voltage in the amp to some value
+ - say 30volts. This is the rail voltage that supplies the output transistors. If the input voltage in the amp is high enough that the gain of the amp times input voltage goes higher then the rail voltage then the output signal becomes flat "clipped". because as the signal amplitude increases the output voltage can't go beyond the rail voltage. So the output becomes distorted and if driven hard enough the output could be almost square wave in shape this is when speakers tend to stop working because it is like hooking your speaker right up to a DC supply or battery. Therefore it melts or pushes the voice coil through the cone
In very simple terms, an amp clips when you over drive it. For example: trying to make a little amp act like a great big amp. Turn it up really loud it will clip.//content.invisioncic.com/y282845/emoticons/cool.gif.3bcaf8f141236c00f8044d07150e34f7.gif