The whole idea of IR (edit: IR=infrared, meaning basically remote temperature measurements) being a proxy for plant health is that when plants have their stomata open, they transpire (evaporate water). Plants do not transpire for nothing, and this means that when the they are transpiring, they are also taking up carbon dioxide (CO2) from the atmosphere. Carbon dioxide uptake is the same as photosynthesis, which is the same as taking up the solar energy that all the living organisms (including themselves) on Earth can use to survive. If the plants are not photosynthesizing, they will close the stomata, and stop transpiring.
In plants, a stoma (in plural stomata) is the place where the transport happens between living and non-living world. For mammals, kind of similar place is the lungs. Understanding the simple principles how they work is key to understanding a lot of things in plant physiology.
When plants are happy, they take up a lot of carbon, but because of fundamental physics (diffusion), at the same time, they invertedly also lose water (it’s evaporation but when it happens through living organisms, it’s called transpiration). Transpiration cools down the surface (leaf), and that can be measured with infrared-measurements. Other things affect the surface temperature as well, but in a somewhat controlled environment with help of simple mathematics that can be taken care of.