An ethical absolute is a command that is true ALL time, in all places and in all situation so there is ONE true morality.
There are eternal moral values applicable everywhere.
Moral absolutism is often identified as a deontological approach - the view that there are certain acts that ought to be carried out and some that ought not. So some things are just right in themselves.
Common perceptions of absolutism correspond with traditional religious morality. So many believe that the absolute law comes from the word of God. However an agnostic or atheist the answer is more complicated they believe that there are some things that just seem a priori in nature.
They fit with Plato world of the forms - there are just something's that we know are wrong without being taught. Do you remember being taught that sleeping with your brother or sister is wrong. So to some extent moral absolutes can be seen as inherent in the nature of man.