A thermal camera or thermographer is a camera which creates an image of what temperature things are. It uses a temperature sensor and sweeps back and forth across a scene to record temperature variation. It represents infrared energy emitted by objects.
The infrared energy is not visible because wavelength is too long to be detected by the human eye.
Everything with a temperature change above absolute zero emits heat, even what we consider as cold objects.
Warmer objects emit more heat, thus more infrared light.
An IR camera detects infrared energy/heat and converts it into an electrical signal. The lens focuses not on visible light but waves from infrared energy onto an infrared sensor array. Thousands of sensors on the array convert the infrared energy into electrical signals. This is then converted to an image and allows you to see how heat is distributed.