A thermographic camera or thermal imaging camera is a device that
forms an image using radiation other than visible light. This is a
thermal image of a chihuahua taken in thermal or infrared light. Notice
this method detects changes in temperature throughout the
chihuahua. An application of thermal imaging using infrared light has
been developed to help homeowners save money. What is this
application?
Thermal images can be used to locate household
pests by an exterminator.
Thermal images pick up body heat and can locate
where people are in the home.
Using thermal imaging to detect heat energy loss
through the roof and windows.
Using thermal imaging to detect exposure to radon
being emitted from the ground.

All warm objects emit heat energy in the form of infrared radiation, which is a kind of invisible light (light with a lower frequency and longer wavelentgh than the visible light).

The amount of heat energy emitted by an object is proportional to its temperature: the higher the temperature of the object body, the greater the amount of radiant energy emitted.

A house can loss heat energy by radiation through the walls, roof and windows.

A thermographic camera or thermal imaging camera will detect the infrared radiation emitted through walls, roof and windows, showing a red image in those areas.

I found the image attached in internet, showing the red areas around the roof and windows of a house, mainly, using a thermal imaging camera.