Infrared Imaging, Infrared Thermography, and Thermal Imaging refer to a process of using thermographic cameras to produce images by detecting radiation in the infrared range of the electromagnetic spectrum. That range is roughly between 900-14000 nanometers.
And here is another way to put this: all types of objects emit infrared light / energy in a form of heat. Because each of the objects / its components as well as parts of our body have slightly different temperature an infrared camera sensor detects those temperature variations and “paints” an image based on those readings. Because the output image is based on radiation, dark or bright environment does not affect the image.