Brake Warning Light

[68 P.O.S.]

Mine turned on after work today. I haven't had time to check it very well yet, but I did find a burned out tail light. I had a co-worker follow me home just in case, but the brakes worked fine with no noticeable difference from normal performance. What does this light exactly warn you of??? Is it for anything specific or is it for the overall braking system as a whole? My brakes, hoses, and cylinders are all <1 yr old so I doubt it is that. Will I need a new prop vale maybe? This may be the perfect excuse to upgrade to power discs! Hopefully my wife will see it that way.