A "final draft" is a
document that is thought by its
author to be in its
final,
publishable form. After a "
rough draft" has gone through a cycle of
review and
revision (usually several such cycles occur), the final draft is the end result. Aside from being free of
spelling and
grammatical errors, a final draft should be as clear and refined as possible.
As this is somewhat subjective, I look at this document state as being slightly mythological, although I suppose it's merely a matter of semantics (a document becomes a final draft once you refuse to revise it any further, for example).