If an internal door is "pre-finished" or "fully-finished", it will have had numerous coats of clear, hard-wearing AC lacquer professionally applied and will arrive ready to hang.
"Unfinished" means the doors will come raw so treatment is required. For information on how to treat or finish you doors please read our post on How to treat internal oak doors.
For more information, please read our post on Pre-finished Internal Doors explained.