exhaust temp is usually used for tuning, mostly for air fuel ratio using ECU or piggyback controllers, fuel pressure regulators; or boost tuning. or can also be an adittional tool for monitoring your engine well being lah.
to use it, 1stly you have to know your normal (ie standard) exhaust temp range. when you know that, any temperature excessively higher than normal is dangerous and can mean something is damaged (fuel pump etc etc). when used for tuning the air-fuel ratio, a lower than normal temperature will mean the engine is running rich, a higher than normal temp indicates the engine is running lean.