The warning light is indicating that a fault exists but not to the point that you have to stop driving. First check that the brake lights are working as if a bulb has blown (or the brake light switch is faulty) then it will show up as a glow plug warning light.

If the lights are okay then we would suggest that you get a diagnostic carried out to see if a fault code exists( ie P ---- 4 numbers) from which you can identify the problem. It could be one of the many sensors or the EGR valve (exhaust gas recycling)