The Rio Poneglyph is integral to the world of One Piece, as it is one of the keys to uncovering the mysteries that are the "True History". The mysterious artifact that is spoken of is not an object per se, but a collection of nine special Poneglyphs that show the complete historical narrative hidden from the world when put together.
Poneglyphs are themselves huge cubic steles created by the Kozuki family in Wano Country. These indestructible stone blocks are inscribed with ancient script that only a few can decipher. According to the World Government, it classifies Poneglyphs as dangerous artifacts, likely because of the secrets they may contain.
According to Tamago, one of the characters in the series, a Rio Poneglyph is composed exactly of nine Poneglyphs. This information is very important in that it provides a concrete number regarding finding the truth pertaining to the history of the world. "Rio" in Rio Poneglyph means "True Text of History" in Japanese:. Once more, this puts an emphasis on the importance of the One Piece narrative.
It should be noted that the Rio Poneglyph is not one of the Road Poneglyphs. While the four Road Poneglyphs point to the location of Laugh Tale, the final island in the Grand Line, the nine Poneglyphs forming the Rio Poneglyph contain the true historical account.
For a concept like the Rio Poneglyph, it actually had a rather early introduction into the series. It was revealed in Episode 194 of the anime that, when the Straw Hat Pirates' archaeologist, Nico Robin, had read the Poneglyphs on the Golden Bell in Skypiea, she finally knew how to get to the Rio Poneglyph. Also, in this episode, it is revealed that the previous Pirate King, Gol D. Roger, was able to read those mysterious stones and left a note behind in that ancient language used by the Poneglyphs.
Being able to read Poneglyphs, Nico Robin becomes a very key character in the quest for Rio Poneglyph. She happens to be the only survivor from Ohara, an island of scholars destroyed by the World Government for researching Poneglyphs, hence bearing the weight of that knowledge and its responsibilities.
Having taken the Straw Hat Pirates to locations all over the Grand Line, the Rio Poneglyph has been a target. From the known Poneglyphs, ones that are said to contribute to the location of the Rio Poneglyph are:
The one on Zou
Another from Big Mom's treasure room
The Poneglyph in Wano at the base of Mount Fuji
The meaning of the Rio Poneglyph is much more than just being part of historical curiosity; it is said to contain information that would shake the very foundations of the One Piece world's order. This may be the reason why the World Government is so keen on preventing this knowledge from being pieced together by anyone.
Ancient Poneglyph script presents a few special challenges. Deciphering these stones not only requires one to read the language but also the context to understand the historical significance of the information contained within them. There have been many speculations and discussions among fans about what the nature of the script is and what kind of methods have been or will be used to decipher it.
In the greater scheme of things in the One Piece storyline, the Rio Poneglyph becomes one of the eventual goals for the protagonists. Presumably, it will show what is contained within: the truth of the Void Century, a period in the history of the world erased from the records. This would have deep repercussions on the present scheme of things and the very nature of the mysterious treasure "One Piece.".