Based on the file naming convention, refers to a specific, modified ROM file for the Nintendo 3DS game Etrian Odyssey Untold: The Millennium Girl .
The file has been "decrypted," meaning the Nintendo 3DS encryption has been removed. This allows the game to be played on PC or Android emulators like Citra , rather than requiring a modified ("hacked") physical 3DS console. EOU-TMG-(UNDUB)-DecrTD-Ziperto.rar
Etrian Odyssey Untold: The Millennium Girl (EOU-TMG). This is a role-playing game developed by Atlus, combining dungeon crawling with a structured story mode. Based on the file naming convention, refers to
This indicates the ROM has been modified to include the original Japanese voice acting while retaining English text. This is highly desired by fans who prefer the original audio over dubbed versions. Etrian Odyssey Untold: The Millennium Girl (EOU-TMG)
This refers to the source website that provides these modified ROMs, which frequently hosts decrypted and undubbed versions of Atlus games.
If you're asking about this for technical or archival reasons, I can also explain: How to What the differences are between this and the original Where to find other similar modified ROMs
A .rar file is a compressed archive containing the .3ds or .cia game file.