A system is at absolute zero when it is at 0 degrees kelvin (0 K).
At absolute zero, the system has no energy. The particles are completely motionless, thus the name “absolute zero”.
Scientists have tried to produce conditions of absolute zero. They have reached a fraction of a kelvin, but never 0K. As of right now, it is impossible to reach absolute zero, but it is still an important concept to understand.
It is also important to understand that 0˚C is not absolute zero. 0˚C = 273.15 K, which is far from absolute zero. In celsius, absolute zero is -273.15 ˚C.
So understand that 0˚C does not mean that a system has no energy/no temperature. However, 0 K, or absolute zero, does.
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