What is "clean" power?
Ideally the AC power that feeds our high performance audio, home theater and studio systems would be rock steady - it would not go up or down in voltage depending on the time of the day and it would not get interrupted by storms or accidents. It would also be free of power line 'nasties' such as transients, noise, distortion and DC offsets.
The chart below shows what clean power might look like - purely sinusoidal AC at 120V RMS. RMS stands for Root Mean Square (RMS) and for a perfect sinewave RMS equals 70% of peak voltage.