(S/N) The ratio between what goes in a device designed to alter or record sound, and what comes out the other end. If what you get out the other end is all distorted or hissy the piece of equipment has a low signal to noise ratio. A very expensive DAT machine for example, will have a high S/N ratio because what you get out will sound very close to what you put in. The same goes for a very expensive digital effects processor, if you want a sound to come out with reverb on it and instead it comes out with reverb and a bunch of hiss you would not be very happy.