In short, it is the technique of using the heat given off by an object to produce a visible image of it.
All objects emit heat (infrared energy). The energy that is emitted is called a "heat signature." The hotter an object is, the more infrared energy it emits. A thermal imager detects that energy, as well as differences in energy between objects, and converts it to an electronic image.
Thermal imaging does not require any light to be present, and is not dependent on ambient light. This means that unlike night vision technology, it can work in complete darkness and can see through smoke, fog, and haze.
Using a thermal imaging device enables the user to see differences in heat to better identify objects when natural vision can't. Heat can't hide, even in the dark.