Thermal imaging is a method of improving the visibility of objects in a dark environment by detecting the objects' infrared radiation and creating an image based on that information.
Here's a brief explanation of how thermal imaging works:
All objects emit infrared energy (heat) as a function of their temperature. The infrared energy emitted by an object is known as its heat signature. In general, the hotter an object is, the more radiation it emits. A thermal camera is essentially a heat sensor that is capable of detecting tiny differences in temperature.
The device collects the infrared radiation from objects in the scene and creates an electronic image based on information about the temperature differences. Because objects are rarely precisely the same temperature as other objects around them, a thermal camera can detect them and they will appear as distinct in a thermal image.