It's referring to the brake lights.
It senses.. I don't know exactly how, but it senses whether a load is present or not. Yes, it is accurate... However.....
We just went through an issue with the sensor on my Dad's '87 Camry.
The brake lights suddenly stoped working, and the light failure warning light was on. Couldn't really figured it out.. replaced the brake pedal switch, didn't fix anything..
So I started messing with the bundle of wires going into the sensor, and they came back on. Worked for a few more weeks, then went off again. This time fiddling with the wires didn't do anything. I thought maybe a wire was frayed inside the insulation or something equally troublesome.
I unhooked the wires, took the sensor out, took the PCB out of the casing and immediately noticed a cold solder joint. Warmed up the iron, bam.. fixed.
The dealership wanted over 100$ for the part. :roll:
Seems strange that it would go on and then go off, if no lights are burned out. I'd also consult your manual for exactly what it means, they may do things differently now.