Like me, you were probably taught in some introductory science course that the minimum temperature for matter of any kind was zero Kelvin, a temperature called 'absolute zero' (corresponds to -273.15 degrees Celsius - slightly colder than a bad day in Winnipeg). The Kelvin scale is based on this minimum measurement.
We often find in science that certain boundaries may be crossed. What Schneider and his colleagues have done is helped coax a gas to sub-absolute-zero temperatures, if only for a short while. As many 20th and 21st century science discoveries, this phenomenon centers around quantum physics, which correctly asserts that things are not quite as they seem. Unlike Newtonian physics, which incorrectly assumes that the state of matter has one absolute value, quantum physics views matter in the way of probability functions. Without getting too deep into that now, let us agree for the moment that quantum physics is bizarre and not intuitive 100% of the time.