what does a power conditioner really do?

It could do many things or very few, but the goal is usually to remove power line noise from the system, thereby improving the low level detail - In theory. In reality, a power regenerator actually takes your wall signal and converts it to a new, clean AC signal, feeding your components with fresh, perfect-voltage AC. Sounds great right? So why doesn’t everyone use them? Then you have the high-end conditioners, usually with some sort of sophisticated filtering to remove the hash between the notes and the overall quietness of the system. Many audiophiles use these. Then you have the high-end power strips that are only expensive outlets, wired together with expensive wire. The best of these types are said by some to render more improvement than either of the others. The only way to really know is to try one and see if it moves you. I will say, a lot has to do with the quality of power coming into your home. If it’s really great, a conditioner might not improve anything that much. But I will say, IMO, even a good basic power strip like the $350 CablePro, can make a sonic improvement in terms of soundstage depth and overall quietness. Last point - some believe that all power conditioners degrade the sound, especially of larger amplifiers and insist on going direct to the wall. So that’s all I have to say about that.

what does a power conditioner really do?

It transforms cheap power into expensive power  ...