We tend to regard cold as the opposite of heat, but that is a misconception: in physical terms there is no such thing as cold, there is only heat - and less heat.
Physicists have spent hundreds of years discussing the exact nature of heat and cold. For a long time, heat was considered an invisible substance in its own right, a so-called "caloric fluid". Another theory stated that combustible substances contained something called "phlogiston".
It wasn't until the late 19th century that the caloric fluid theory made way for an understanding of heat as a form of energy - or, put differently: heat turned out to be the motion of atoms in matter. The colder something is, the less the atoms move.
What is absolute zero?
Theoretically atoms would stop moving at -273.15°C. This temperature is called absolute zero because nothing can ever be colder than this (since there can't be less motion than no motion). But even absolute zero is physically impossible, as there is always some motion and therefore some heat around - even the coldest places in the universe are at least 1°C above this temperature.
Absolute zero defines the low end of the kelvin temperature scale: an object's temperature in kelvin describes how much warmer the object is than absolute zero.
How close can you get to absolute zero?
Over the past hundred years, scientists have produced temperatures that came very close to absolute zero -0.000000001°K (or one billionth of a kelvin) is not uncommon.