A Radiator is an object that'll heat up your prison. It currently does not require an electrical connection.
After Update 5 radiators need hot water to operate.
Hot water comes from a Boiler ,which requires regular water to function.
Radiators will be become less effective when their connected hot water pipe is far away from the Boiler.
In order to obtain the Radiator in-game you need the temperature option selected in the create a new prison page.