A surface that becomes colder than the surrounding air, in direct sunlight, without any energy input, sounds like it should violate something. It does not. It exploits a specific gap in how the atmosphere absorbs infrared radiation.
The physics in short
Everything radiates heat. Normally that radiation is absorbed by the atmosphere and comes back, which is why the ground does not simply freeze at night.
But the atmosphere is transparent in a narrow band of infrared wavelengths, roughly between eight and thirteen micrometres. Radiation emitted in that window escapes to space, and space is very cold.
A material engineered to reflect nearly all sunlight while emitting strongly in exactly that window loses more heat than it gains. The result is a surface several degrees below ambient temperature, in daylight, permanently, with no moving parts.
Where it is useful
Roofs, most obviously. A coating that keeps a building cooler reduces air conditioning load, which matters most in hot climates where electricity is expensive and grids are strained precisely when demand peaks.
Also refrigeration in places without reliable power, condensation of drinking water from air, and cooling for solar panels, which lose efficiency as they heat up.
Why it is not everywhere yet
Humidity. Water vapour absorbs in the same infrared window that the effect depends on, so performance drops substantially in humid air. The technology works best in dry climates, which overlaps only partly with where cooling is most needed.
Durability is the other issue. The optical properties depend on precise surface structure, and surfaces outdoors accumulate dust, degrade under ultraviolet light, and get rained on. A coating that works beautifully in a laboratory and for eighteen months on a roof is not a product.
Where it stands
Commercial coatings exist and are being applied at increasing scale. The performance claims worth trusting are the ones measured over years rather than the peak figures from a clear desert night, which is where most of the impressive numbers come from.