Raftel is the island at the end of the Grand Line which supposedly holds the treasure One Piece.
It is also said that upon reaching Raftel, if one has taken with them along their journey the passages of the Poneglyphs, then the Rio Poneglyph will finally be revealed. For this reason, Raftel is possibly the final destination, at least for Nico Robin.
Gol D. Roger and his crew are the only known people to have reached the island. Roger offered Whitebeard the information on how to get to Raftel, but Whitebeard declined because of his own dreams of achieving a "family" of crew members instead of reaching One Piece. This also indicates that getting to the island is not as simple as sailing to the end of the Grand Line.