What is absolute zero?

The temperature of an object is a measure of how much energy it has, and when something has energy the atoms in that object vibrate. The hotter it is the more they vibrate, the colder it is the less they vibrate. As you lower the temperature those vibrations reduce. As you get towards what we call absolute zero, that's when the vibration has almost completely stopped, so you can't get lower than -273 degrees, that's zero degrees Kelvin, matter has not energy or motion at this temperature, hence it's absolute zero.