i test drove a 1994 XJS convertible today and this light came on at dash every time i stepped on brakes- what does this mean?

 

If you have the handbrake on, the engine running, and press the brakes, this light illuminates, which indicates the bulb failure warning light system is working. If the handbrake is off, the engine is running, and the handbrake warning light switch is adjusted correctly, this light will illuminate if you have a brake light out. It will also illuminate for about 20 seconds when you turn the lights on and will stay on if you have a bulb out.
So either your handbrake is not fully off, or you have a brake light out, or the handbrake microswitch is out of adjustment and the circuit thinks the handbrake is on when it is not.

 

That is simply the 'bulb out' message. As the Tester only works when something is on, it means that when you press the brake, it notices one of the brake lights isn't working. If you reverse up to a Wall and press the brake, you will be able to see what is going on.