In the context of the Cemu Wii U emulator, the file is a configuration file used to decrypt and run games in (disc image) formats. Cemu Emulator Essential Review of keys.txt
Individual keys for each specific game disc or digital download. You can use homebrew tools like wii u keystxt for cemu
To install updates or DLC, all three keys must be present in your keys.txt. If you install an update and Cemu crashes or freezes, check that the update's specific Title ID (note the E or C in the 8th digit) has a valid key. In the context of the Cemu Wii U
Now, Cemu—the Wii U emulator on his gaming PC—was refusing to cooperate. He had the game files, the updates, even the DLC. But without the keys.txt file, they were just encrypted gibberish. If you install an update and Cemu crashes