What does the inital /dⁿ/ stand for?
It's the IPA diacritic for the nasal release from a stopped consonant:
Quoting right from that Wikipedia page:

That is, the /d/ is released directly into the /n/: [ˈsʌdⁿn̩].

Where [ˈsʌdⁿn̩] is the IPA transcription of the English word &quot;sudden&quot;, where the blocked air flow from the articulation of /d/ is released through the nose (you should feel it vibrating in your pharynx).
For those who don't have the font installed, I believe the character in the OP is: