Both thermostatic mixing valves and just plain mixing valves control the hot and cold water coming into the shower faucet.
But the difference between a mixing valve and a thermostatic mixing valve is the thermostatic mixing valve senses and controls the actual water temperature coming into the shower, while a mixing valve also known as the pressure-balance valve senses and controls only the ratio of hot water to cold.
The thermostatic mixing valve keeps you from getting either too cold of a shower or too hot of a shower should someone start a washing machine or flush the toilet etc.