"I open the book."
Translation:Saya membuka buku itu.
Hai teman! The use of "tersebut" is correct in this sentence. It is translated as "the" / "that" / "those". And yes, the Indonesian meaning would be something like "what is said" or "which is mentioned", but I am not really sure about the literal translation here. So, I prefer not to think of that, though I would say it is interesting indeed. Perhaps, I think it is good to know a different sentence construction, or different uses of "tersebut", but this is still very advanced for me. That I have said is as much as I know.
I hope it helps.
Selamat belajar! :)