[Addendum] THD+N is a measure of the purity of the output i.e how well the amplified audio signal matches the smaller input audio signal that's fed into the amplifier (from a CD player for example). So the lower the number the better. Typically, the amplifier circuit itself adds some distortions and noise to the actual audio signal. For example, if you fed a pure 1V 1kHz sine signal into the amplifier, your output might be a 20V 1kHz sine wave but with additional signals mixed in. These signals will include inherent noise generated by the amplifier and harmonics generated by the switching nature of the Class-D amplifer (if you look at your audio signal at the output of the MOSFET switches and before the low pass filter, it will look like a square wave or more specifically, a PWM).