Quadrophonic systems have four speakers. "Balance" therefore means you have to handle front to back as well as left to right.
The "balance" control on this system looks like a joystick.
The display above it is an edge lit plexiglas picture. It is static (doesn't change.)
The red spot is an LED behind the plexiglas. It is mechanically coupled to the joystick balance control below the display.
This gave you a snazzy looking image of how the balance was set - but doesn't really tell you anything more than the position of the joystick. It just looks way cooler.
All of the displays on that system appear to be edge lit
plexiglas with mechanical elements in the background. The tuner is like that - edge lit dial, mechanical pointer.
It looks like some futuristic CRT or custom vacuum fluorescent display, but was much cheaper to produce back in the 1970s. The manufacturers were trying to look like science fiction movies or NASA stuff, but having to do it cheap enough to sell.