It is an insight to III. Sora makes his decision about King Mickey's letter which involves some journey. (You never actually see the writing) Kairi wishes the two of them (Riku goes too) good luck since she is not going. Before this happens, Terra, Ven, and Aqua, having lost their bodies, souls, hearts, or whatever are talking to people. Ven with Terra, and Aqua with Ansem I believe. While they talk, since time has gone by since they became in the state they are (which I never truly understood) realize only one person can save them and the connections of everywhere. all muttering the same name, they understand their light will be returned. The person is Sora.