Good temperature control is one of the main technologies that separates professional brewers and homebrewers. While the pros have access to jacketed cylindroconicals that can hold fermentation within a very narrow temperature window, most hobbyists improvise some kind of controlled, insulated environment. And the brain of that environment is the temperature controller.
A temperature controller is a glorified on/off switch with a sensor. Plug the controller into the wall, plug a refrigerator or freezer into the controller, and place the temperature sensor within said refrigerator or freezer. Then just set the temperature you want, and let the controller power the cold box on and off to maintain it. It’s a convenient way to override the internal thermostat.
Here are a few things to consider when buying a temperature controller.