Simple Definition:

Absolute zero is the lowest possible temperature, at which all molecular motion stops. It is equal to -273.15 degrees Celsius (-459.67 degrees Fahrenheit).

For example, when water freezes, it reaches a temperature of 0 degrees Celsius (32 degrees Fahrenheit). If the temperature drops even further, it may eventually reach absolute zero, where no further cooling is possible.

Very Simple Definition:

Absolute zero is the coldest possible temperature, where everything completely stops moving. It is colder than anything we can feel.