A TUID is a "tads.org Unique Identifier" - a serial number that uniquely identifies a game listing or member profile in our database. A TUID has no meaning outside of our database, but we show them in the game listing because you can use them to write permanent, stable references to listings in our database. The database is designed so that once a TUID is assigned to a game, that identifier will stick with the game for as long as it's in our database.
You can find the TUID for a game on its home page, in the Details section.
The format of a TUID is a mix of random letters and numbers, 16 characters long. We could have just used a simple sequential number as the ID (this is how Baf's Guide works), but we decided to use a long string of gibberish instead for a reason: these sorts of identifiers easily stand out from the crowd in full-text searches.