The Codex window functions as the in-game encyclopedia. Not all the information is displayed from the beginning; instead, it only displays information for items that have been discovered by the player. It contains three tabs: Inbox, Tutorials, and Recipes.
The inbox tab contains a log of messages from ADA, such as discoveries, milestone and research completion, etc. Clicking the 'play' button will playback the voice message from ADA.
The tutorial tab contains a log of tutorial messages from ADA, such as from dismantling the drop pod, building the HUB, etc. Clicking the 'play' button will playback the voice message from ADA.
The recipes tab can be used to view crafting recipes of all currently craftable/buildable items. Only items that can be currently crafted/built are shown (e.g. the recipe for Computers is only shown after unlocking the Tier 5 milestone: Industrial Manufacturing).