Clipping is taking the electrical signal and pusing it beyond the abilities of the component. A speaker works by alternating the positive and negative back and forth to move the speaker and make sound. (Alternating current or AC) Using an oscilloscope you will see that the signal is a smooth curve (wave) that moves up and down. Clipping is taking the signal and putting a flat spot in the curve. ie DC current. When DC occurs during the AC power wave it is "clipping" the peaks and valleys off the wave.
How this affects speakers is that the speaker will get to the power waves clipping point. (Say on the peak) Then the DC kicks in and "kicks" the speaker sideways. Then when AC continues on the wave, the speaker's voice coil is still kicked sideways and grinds down the pole and magnet structure's wall. Then at the other end of the wave (the valley), the clip kicks in again and kicks the speaker the other way. AC continues and grinds the voice coil up the other side. Now do this at 50Hz and now your speaker is grinding the voice coil 100 times per second. Bye Bye speaker.
