In the context of games, especially those with a focus on storytelling and world-building like Resident Evil, a "codex" often refers to an in-game collection of information. This can include lore about the game's universe, character bios, and item descriptions.
The phrase "Verified" is critical. By December 18, 2019, many unofficial mirrors were hosting fake "CODEX updates" that contained malware, keyloggers, or simply broken patches that caused the game to crash at the Raccoon City police hall lobby. Resident Evil 2 Remake: Analyzing the 20191218 Update
If you have an existing FitGirl repack of RE2 from early 2019 and wish to manually apply the 12/18/2019 CODEX update: Ghost Survivors DLC fixes General stability & performance