The basic concept is that since heat is a result of the motion of the molecules in an object and that this motion causes those molecules to move around and spread out causing a condition of high entropy.
As an object is cooled the object molecules slow down allowing the forces between molecules to organize the molecules. At absolute zero all of the heat is removed and the molecules stop moving allowing the forces between molecules to fully organize the molecules resulting in zero entropy.
Absolute Zero is the lowest possible limit on temperature, it is refered to as 0 Kelvin, which is -273.15 oC (-459.67 oF),
While absolute zero is the lowest limit on temperature, in practice it is not actually achievable. You can get infinitely close but not exactly at absolute zero but it is impossible get rid of that last bit of heat.