If you push a signal past 0 with a digital mixer you clip the signal. If you look at the result of this you can see that the top of the waveform has been squared off - information has been lost and it usually sounds horrible. But if used properly it is just a type of distortion.

You won't notice the problems of software clipping until you record it back digitally. Its when you need to capture the sound that you notice that you can't record all the extra volume you are throwing at it.