The coldest temperature possible in the universe is minus 273 degrees on the Celsius scale, also known as 0 degrees on the Kelvin scale. At this point matter basically freezes, and all motion of atoms and molecules stops.
Not surprisingly, it is extremely hard to create temperatures this cold, but scientists today can get within a half of a degree of it. When cooled to near this coldest limit, certain metals began to show some remarkable properties - they started to -superconduct' - that is, allow electrons to pass through them with no resistance at all. 'Brilliant', the optimists said, 'within 10 years we'll have 100% efficiency, levitating trains and a better world'. That was in 1911, and so far, few of these things have taken off (apart from the odd prototype of levitating trains). It turns out that superconductors at these extremely cold temperatures are very expensive to set up.
So for many years the search has been on for superconductors that can work at warmer temperatures - that's warmer relative to 0 degrees Kelvin, still not warm enough to be running around in your undies.