Usually, when you average an AC waveform you get exactly 0V, which means the
wave is symmetrical about 0V. There might be some components esp. opamps and
amplifiers, which do add a DC-component to the output. Whereas a few
millivolts are usually of no concern and normal, an extended amount(Volts)
might harm loudspeakers. You can see the diaphragm going off center and
staying there, so the excursion is assymmetrical. This causes distortion and
heating of the voicecoil. A DC-offset can be blocked with a capacitor.
DC offset is a average (voltage or current) of some waveform. DC offset
causes constant flow of current. It may cause damage for some circuits,
because constant current is non-essential in many applications, yet it
produces a heat according to Joule-Lenz law. DC offset may also cause a
breakdown for transistors gates or diodes (of course it it's to high -
above breakdown voltage).