In the case of an analog signal, clipping occurs when the signal is driven beyond its limits causing distortion.
In the case of a digital signal, clipping causes the tops of the peaks to be 'clipped' off, which results in distortion.
Clipping occurs when an amplifier is driven into an overload condition. Usually the clipped waveform contains an excess of high-frequency energy, so that the sound becomes hard and edgy.
Hard clipping is the most frequent cause of "burned out" tweeters; even a low-powered amplifier or receiver driven into clipping can damage tweeters which would otherwise last virtually forever.
The clipping level of a microphone is the maximum output signal (measured in dBV or dBu) that the mic can produce before the signal becomes distorted.