Like the Gen1+, Gen2+ and Gen3 image intensifiers that are at the heart of night vision, thermal imagers depend on sensors (called focal plane arrays) which cleverly convert wavelengths of heat energy that are invisible to our eyes into something we can see.
In fact thermal imagers don't "see" anything. Rather, they detect minute differences in heat - as small as 0.01°C - which they display as a monochrome shade of grey on their internal black and white video monitors.
The heat that is detected by thermal sensors generally comes from one of two sources. The first, referred to as "heat signatures", are actively generated by objects such as warm blooded animals, engines and machinery. The second source of heat energy sensed by thermal imagers comes from inanimate objects such as rocks, buoys and vegetation which passively release the heat they absorb from the sun at different rates.