Basically John Deacon built a small amp from the parts of a stereo, and powered by a 9 volt battery.
Brian used it and found that he could overdrive it to such an extent that it created some rather pleasing tones, due to it's size the tones that come out of it tend to sound like trumpets and horns, most of Good Company was put together using it.
Brian never uses it on stage however you can now buy a VOX version that does a great job and also to new Brian May pedal emulates the sound well.
Think of the classic Brian harmony sounds such as God Save the Queen and you have it.
"It is better to sit in silence and have people think you're a fool, then to open your mouth and remove all doubt"