Thieves' Cant is a language automatically taught in its entirety to players upon joining the Thieves' Guild. There is both a spoken and written form, although both those terms are used loosely. Non-thieves cannot understand the language, nor can it be learned anywhere.
Kordane teaches you the secret squiggles, gestures and finger-wiggles of Thieves' Cant.
In its written form the Cant is a tactile, Braille-like language. The Thieves' Guild Licence is the primary place a player will encounter it. The quota on the Licence can be "read" normally in a room with sufficient light or "check"ed with a free hand in rooms of any brightness.
Letting your fingers slide over the Thieves' Cant on the licence, you read the following:
When in a room with those speaking Cant, players without the language will see blank lines. Thieves' Cant is affected by accents, as is standard for Disc languages.