What is a TRV (thermostatic radiator valve)?---an explanation for householders.

TRVs sense the air temperature around them and regulate the flow of water through the radiators which they are fitted to. They do not control the boiler.
They should be set at a level that gives you the room temperature you want. These settings may have to be different in each room, and you should set the TRVs to suit each room and then leave them to do their job.
Turning a TRV to a higher setting will not make the room heat up any faster. How quickly the room heats up depends on the boiler size and setting, and the radiator size. Turning a TRV to a lower setting will result in the room being controlled at a lower temperature, and saves energy.
TRVs need a free flow of air to sense the temperature, so they must not be covered by curtains or blocked by furniture.
TRVs cannot turn off the boiler when the whole house is warm. To do that, you will need a room thermostat as well. The radiator in the room with the room thermostat should not normally have a TRV, but, if it does, keep the TRV on the maximum setting and adjust the room thermostat as explained with the instructions.