Open the menu by touching the button on the top right of your screen. You can find the Codex tab in the center of the menu screen.
The Codex contains various details about anything you encounter in game, such as equipment, monsters, gatherable items, collectables, and more.
When you obtain a new item, or defeat a monster you haven't met before, the Codex is updated with the new information.
※ The Monster Codex is categorized by region, and the Transformation Codex is updated when you actually transform!
Once you collected enough of a type of Codes entries, you unlock rewards which you can collect via the Codex screen
Why don’t you try and fill in the missing parts of your Codex?