Codex means in the bible: An ancient manuscript volume containing biblical text. The codex manuscript style is similar to our current book format, with pages being piled together and then joined on the one side. This is usually done with sewing. Contrast this with a scroll where the pages are piled on top of each other and joined at one side, usually using stitching. It is easier to use the codex than a scroll. Scholars agree that Hebrews didn’t use codexes before the second or third century A.D. While many documents from the Old Testament are scrolls and some are also codices, they are both scrolls. Three rings were joined on the one side of the codex gold plates that contained the Book of Mormon.
(in Bible Dictionary)