The hero's journey sometimes includes a "father quest." After many trials and ordeals, the hero finds his father and becomes "at-one" with him. This process is called "atonement."
Luke has tried to follow in his father's footsteps as a heroic pilot and Jedi Knight. The dark, unknown side of his father--and of himself--is now unveiled as Luke confronts Vader in the dark byways of Cloud City. Vader reveals to Luke that he is his father. Luke realizes that he must sacrifice himself, rather than become a tool of evil like Vader.
Leia rescues Luke as he falls from the underside of Cloud City, and when Vader calls to Luke through the Force, Luke acknowledges him as "Father"--they have begun to move toward reconciliation. Luke has recognized the dark side of himself as part of his destiny, and Darth Vader has begun his own journey toward transformation.