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).