Sunlight lets you see things. The Sun emits different wavelengths, and the visible region consists of the wavelengths, from about 400 nm to about 700 nm. One of the regions, which is outside of the 400 nm - 700 nm region, that is the region with wavelengths above 700 nm, called near-infrared, is important as well. We cannot see the near-infrared wavelengths emitted by the Sun.

We see leaves as green. The pigments in leaves reflect green and infrared wavelengths of the electromagnetic spectrum. Leaves absorb the other wavelengths in the visible region: red and blue. Compared to unhealthy leaves, healthy leaves reflect more near-infrared wavelengths.

Instruments on satellites can sense both visible and near-infrared wavelengths. A sensor captures strengths of different wave lengths, absorbed and reflected by leaves, as numbers. These numbers, in other words brightness values, can tell us whether the plant is healthy or not.