Thermostat is a temperature sensitive device. It places between the engine and the radiator. The purpose of the thermostat is to maintain optimum temperature of the engine to draw maximum efficiency of the system. 
How will it work is, it blocks the flow of coolant to the radiator till engine gets enough warm. When the engine is cold, no coolant flows through the engine. So it sits wait till engine has reached operating temperature and allows the coolant to flow through the radiator to maintain that temperature.

How it designed is, it’s a small cylinder filled with wax that begans to melt at 180 C ( tipical ). And a rod connected to the valve presses into the wax. When the wax melts, it expands and pushes the rod out of cylinder, opening the valve.