what is a ground lift switch.

Idea is probably to lift the ground and thus counter and hopefully eliminate a ground loop hum, if you have one in the system. I've got them on my Sonogy solid state amps in my system. I found it works to eliminate most if not all of any ground loop hum; have not noticed any negative effects, though the Sonogy amps are only used below 220Hz.

one of my amps has a switch on the back.it say lift.what does this accomplish.are they worthwhile.do they affect sound quality?

Just to expand on Rcprince response. The switch lifts the signal ground from the chassis. It does not lift, disable, the safety equipment ground from the case, chassis, of the equipment.

thank you