The thermal relay is a device that is responsible for protecting electrical motors from possible irregularities. The most common is the overheating of the electric motor. When the engine locks its shaft or is working very heavily, it requires more mains current to try to compensate for the required weight, so the engine ends up having to work with specifications that are not suitable for it. This can damage your coils, causing it to heat up and possibly melting your insulation, and this will close a possible internal short circuit.
At the moment that the possible heating is occurring, the relay will come into action alemite and when this overheating the bimetallic plates of different temperature coefficients heat up, will occur to deformation of the blades and causing them to activate the relay, disarming the motor circuit, as well as the control circuit through its auxiliary contacts.
This relay is an excellent protection component, because after it is activated, it locks up preventing the motor from being turned on again. In this way the motor can only be started when a manual reset action occurs.