Remote sensing instruments are like cameras on your mobile phones. Most take photographs of the electromagnetic energy we can see – called the visible spectrum, but some sensors can take photographs of the energy we can't see.
Energy can change when it hits normal matter like air, water or even your hand. It may slow down, be reflected or be absorbed and re-emitted as a different kind of radiation. This change in energy is called 'attenuation'.
The Sun is the source of most of the electromagnetic energy we see on Earth. The surface of the Sun is approximately 6000 Kelvin which means that the peak energy emitted by the Sun is in the visible wavelengths. As the energy travels through the atmosphere some energy is lost (it is absorbed or scattered). When the radiation comes in contact with an object, such as a tree, some energy is absorbed and some energy is reflected. More energy is lost as the radiation travels back through the atmosphere where it is captured by a remote sensing instrument.