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Absolute zero is a hypothetical energetic state of matter that consists of an object with it's smallest possible value of entropy (which could possibly be 0, neglecting oddities like glasses). Entropy is a quantity that describes the uncertainty in a system's physical state, which generally increases with the system's energy content as more energy means more ability to fly around and do weird shit. Because of this, generally speaking, this minimal entropic state corresponds to the object having 0 kinetic energy, meaning that every subatomic particle of every atom is held perfectly still; there is 0 motion anywhere in the object and it is perfectly frozen. Again, there are exceptions for objects with weird crystalline structures that admit some uncertainty even in a state of 0 motion but those dont count for these purposes. Temperature is nothing more than a measurement of the average kinetic energy of particles in the object. The thermometer is measuring how hard they are physically slapping against the detector as they move around or vibrate in place (on average). Since this average motion is 0 at absolute zero, the temperature is also 0. Hence the name "absolute zero." It's called ABSOLUTE zero because it's not a relative measurement, i.e. it's not 0 degrees above the freezing point of water. It's zero energy. This is also why we don't say "degrees Kelvin," (if you do, you are simply wrong), because the word "degrees" like "degrees C" or "degrees F" implies that the measurement is relative to a baseline. We simply say "Kelvin," as in "273 Kelvin" and not "273 degrees Kelvin."
