I'm having trouble understanding the idea of an algebra using everybody's favorite example, the monoid. What I want, to clarify, is to get some intuition on characterizing the algebra of the free-forgetful monoid adjunction.

 

If F is the free monoid functor, and G is its right adjoint, then G . F is our monad on SET; and its unit eta is a natural transformation taking every set X to the set (G . F) X ("set of words on the alphabet X") in such a way that every element x in X is mapped to its "singleton word" [x]. "Insertion of generators."

 

I'm having trouble, on the other hand, understanding how an algebra could establish a natural transformation between the set (G . F) X for any set X, back to X itself. How would those morphisms map the elements of (G . F) X ? Aren't these algebras supposed to "represent" the various monoids on X? But it isn't generally true that a monoid operation maps back to the set of *generators*. I know I'm missing something here, but what is it? Clarify these natural transformations for me, in "mundane" "baby" monoid-describing language!