Absolute zero or zero kelvin is the minimum possible temperature that can be attained in the universe. At absolute zero, the volume of an ideal gas approaches zero. Temperature is the measure of the average heat in a substance. A substance at any temperature (other than absolute zero) consists of molecules that are in motion. Higher is the temperature, greater is the motion. As the temperature is reduced, the extent to which the molecules or atoms vibrate decreases.
Entropy is the measure of randomness or disorder in a system. Both entropy and heat are closely related. When you heat liquid water, it turns into water vapor, which is a gas. In a gas, the particles can move more freely as compared to a liquid. Therefore, the entropy of gases are much larger than those of liquids.
The third law of thermodynamics states that the entropy of a perfect crystal becomes zero at absolute zero temperature. Does it mean that the molecular motion completely ceases or stops at zero kelvin? Well, no. Quantum physics does not allow that to happen.
According to the uncertainty principle, one cannot accurately determine the position and momentum of a particle with total precision. So, the particles must still move even at the lowest possible temperature. Therefore, particles still possess some energy at absolute zero.
Near absolute zero temperature, molecules start to behave weirdly and exhibit various exotic states of matter. Some examples are Bose-Einstein condensate (BEC), fermionic condensate, etc. When a gas of bosons (photons, gluons, etc) is cooled down to extremely low temperatures, it forms BEC. Under such conditions, large number of bosons occupy the lowest possible quantum state and act as a single particle. Therefore, quantum laws become valid at the macroscopic scale.
Fermionic condensate is similar to BEC but it contains fermions (electrons, protons, etc) cooled down to near absolute zero temperatures instead of bosons. Therefore, it appears as if molecules cease their individual chaotic motion and start behaving as a collective body close to absolute zero.
Attaining absolute zero is practically impossible. The laws of thermodynamics would not allow this to happen because the temperature of the substance being cooled approaches the temperature of the cooling agent asymptotically. But experiments have been successful in attaining temperatures very close to 0 K through the use of cryocoolers, laser cooling, etc.
Temperatures as low as 0.0000005 K have been attained in laboratories on Earth! Outside laboratories, the coldest observed temperature in the universe is 1 K (Boomerang Nebula).
Reaching absolute zero is impossible but the laws of physics still help us to imagine what would happen under such extreme conditions. Hope you had fun reading this article. If you have any suggestions/doubts, feel free to ask them in the comments!