When stuff wiggles between 20 times per second up to 20,000 times per second, our ears perceive this as 'sound'. The number of times that something wiggles (in one second) is called 'frequency', The term 'frequency response' is an abbreviation of 'amplitude versus frequency response'. This is a measure of transfer function, or, how well a device (loudspeaker for instance) conveys all of the frequencies that are asked of it to reproduce. If the device is perfect, all frequencies are conveyed equally (equally in amplitude) from the lowest pitch we can hear (20Hertz) up to the highest (20,000Hertz). The amplitude, quantified by decibels, is then expressed as a plus or minus deviation from perfect reproduction (zero). Most specifications that you see conveniently omit this plus or minus figure, rendering the specification meaningless, by exaggerating the response to be perfect, which it isn't, ever. A real honest spec for a speaker might be, "55 to 18,000 + or - 3dB. This would be a VERY good speaker. Sound stage is not something I'm an expert at, but it is a very subjective description audiophiles like to use to try and make you believe that they can hear something that you cannot due to their 'golden' ears.