Your engine is cooled down by coolant. But for som reason if engine temperature is not controlled and goes beyond the preset ranges the high temperature warning light appears.

There are a number of reasons including the closed or stuck thermostat. The thermostat plays the critical role in cooling system. When the engine is hot enough then the termostat vavle is open letting hot coolant circulates through the radiator. The radiator then cools down the hot coolant and send it back to the engine. However, if thermostat is stuck closed, then hot coolant just can not reach the radiator for cooling, thus hot coolant is contineously circulated in and out of the engine.