A thermo-switch opens and closes according to pre-set temperature levels. Some are mechanical, others are electrical. It may be designed to switch off when temperature rises above a certain level, or it can be made to switch on, when the temperature reaches a certain level.
Heat switches can operate on the bimetallic strip principle. It consists of two different metals or alloys attached back-to-back. As different metals and alloys heat and cool, they expand, and contract, differently. That means that if they are joined, and then heated, the faster expansion of one will force the whole strip into a curved shape.
As the strip changes shape, it can be designed to complete a circuit, and a resulting electrical signal can then do a range of tasks, or it might have a mechanical effect, simply opening a passageway.
Cooling then produces the opposite effect. Breaking the circuit, and closing the passage.