Ok, here’s the technical bit!

Infrared radiation in the middle to far Infra-Red (around 850 to 1400 nanometres wavelength) is emitted by all objects with a temperature above absolute zero (black body principle).

Since the amount of radiation emitted by an object increases with temperature, thermal imaging (infra-red imaging), enables visualisation of variations in temperature. When viewed through our thermal imaging camera, warm objects stand out against cooler backgrounds.

See that wasn’t so bad.