Basically, exactly what it says. The Nerevarine from Morrowind replaces the Dovahkiin in Skyrim. The Nerevarine has access to all items, armors, weapons, spells, abilities, and knowledge he has from Morrowind. He starts the journey the same way the Dovahkiin does, and goes through every quest in Skyrim. Also, in terms of mindset assume he has a crippling hatred of all things daedra. Oh, and books. He is determined to get the best possible ending for everyone.