I found a good explanation here:
(it is from purepower and they have some marketing there, so make abstraction of it :)) )
A power conditioner is essentially a filter, which passes the 60Hz ac from the wall to the audio system, while attenuating to some degree the noise and other spectral impurities that may be present. Some of them also provide outlets that are isolated from each other to some degree, so that noise from digital components can be prevented from coupling into analog components via their power connections.
You should also look here.
There are 10 things that can go wrong with your inbound AC power from the local power company that will affect your system. We call them the ten Power Gremlins.
1. Blackouts: A total loss of utility power
2. Frequency variation: A change in frequency stability
3. Transients: Instantaneous high voltage increase in the range of nanoseconds
4. Harmonics: Distortion of the normal waveform, generally transmitted by non-linear loads
5. Over Voltage:Increased line voltage for extended periods of a few minutes to a few days
6. Brownouts: Reduced line voltage for extended periods of a few minutes to a few days
7. Sags: Short term low voltage
8. EMI (Electro Magnetic Interference):
High Frequency waveform - often referred to as noise
9. RFI (Radio Frequency Interference):
Also a High Frequency waveform - often referred to as noise
10. Surges: Short term high voltage
This is the term used to describe the process of converting the inbound AC from the utility company into DC, then back to AC again as a perfect, noise-free, stable voltage sine wave. Power Regenerators deliver newly generated AC to your system, at the point of use and always keeps it totally isolated from the utility supply.
Edited by dan.gheorghe - 9/5/13 at 5:33am