According to the temperature scale, the least temperature that we can ever achieve in our practical life is - 273°C. This is the temperature from where our kelvin scale of temperature or absolute temperature starts.
The temperature of a substance is the practical manifestation of the internal energy of the substance. And this internal energy is actually reserved in form of molecular vibrations of the substance. With increasing heat, the temperature of the substance increase because of the increase in molecular vibrations of the substance. With decrease in temperature, the vibrations lessen untill - 273°C or 0K where they become completely still. So cooling beyond that point isn't possible. So at - 272°C or 1K, the molecules will be very lightly vibrating and they will have the least possible internal energy.
