The third Law of Thermodynamics states that the entropy of a perfect crystal is 0 if it is in absolute zero temperature. Absolute zero is the lowest possible temperature, but it is theoretic, as achieving it isimpossible with thermodynamical methods. The problem is that any objects near to it would be heating it up. It is -237.15 degrees Celcius and the coldest humans have ever reached is one billionth degree above absolute zero. Another property of absolute zero is that all matter that reaches it will have an enthalpy of 0. Enthalpy is in basic terms a measure of heat in a system. It would be calculated by in which PV is the volume and pressure, H is enthalpy and U stands for the internal energy. Bacically, at absolute zero molecules stop moving completely aand this causes perfect order within them. The uses of this law are very limited, but it provides the basis for the other two basic laws of thermodynamics. The most important part that it tells us is that it is impossible to reach absolute zero. And that itself is important as it tells us that everything in the universe is moving.
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