That means that with an input signal of Zero dBm you would get 100watts output into 8 ohms from the amp, just a reference level to help you match the input level to the optimum point for the amplifier to produce the specified output that it should. Likewise, more than 0 dBm input will yield more than 100 watts at 8 ohm output. It's there way of putting the input sensitivity value on the back of the amp. The actual power value of 0 dBm depends on the specified impedance. I can see how that can be a confusing label above the input, but long story short, it's not saying that it accepts a 100 watt at 8 ohm input signal.