A thermostat is a part of an engine cooling system. The thermostat helps to maintain the engine coolant temperature within the normal operating range.
How it works:
When the engine started cold, the thermostat is closed. The engine coolant (antifreeze) does not circulate through the radiator (see the pictures). Instead it returns back into the engine through the by-pass tube. This helps to warm up the engine faster.
As soon as the engine temperature reaches the normal range, the thermostat opens and the engine coolant starts circulating through the radiator where it cools down.
If the coolant temperature in the radiator drops too low, for example when driving on a freeway in a cold weather, the thermostat closes again to keep the temperature within operating range.
Thermostat problems are common. Defective thermostat must be replaced - luckily, it's fairly inexpensive part.
Thermostat stuck open
Thermostat stuck closed