Within the first few paragraphs of this novel, we find out that Orlando wants to write. This is going to be the one trait that unifies this character though all of his changes of age, gender, even social status. The first act of a plot is the point of no return, and it's this detail about Orlando that sets the course of the novel.
Orlando, Version 1.0, has failed by Chapter 3: he's been disillusioned with love, poetry, and even sex. Orlando has accepted exactly the kind of public post (Ambassador to Constantinople) that makes him all surface and no substance − we see him appear in public, but we don't know anything else about him. The biographer stops trying to present Orlando's inner life and starts recording him through the eyes of other people. In short, this is the moment when Orlando is furthest from his goal, and he needs to do something drastic to regain his confidence with words and his pleasure in writing. So he becomes a woman.
The remainder of Orlando chronicles Orlando's work to become a better writer. She learns to stick to social codes just enough to get by, but not so much that she loses her distinctive voice.
As we watch Orlando building herself up from scratch, we start to learn her own private vocabulary. Like, her house and its many rooms represents her personal lineage. Shel represents her own essential nature. (Shel's "Character Analysis" for more on this.) Queen Elizabeth, Orlando's first patron and supporter of most of the great Renaissance writers represents the English literary tradition. Obviously, this is a bit of a simplification, but each of these symbols takes on these meanings through the course of the novel. And it's this private code that brings us into Orlando's inner landscape by the end of the book.