Parking brake dash light

[Tucson38]

Question about the parking brake warning light on the dash ('85 GMC). Does this light only indicate when the parking brake is on or can it indicate a problem with the main braking system?

Mine starting lighting up and turning off randomly yesterday although the parking brake was off. I'm concerned that it could indicate a fault in the main braking system or does it only indicate that the parking brake is on? I checked the fluid level, it was normal, no leaks anywhere.