A trip is just the short form for a tripcode. From the 4chan FAQ,
Tripcodes can help verify a user's identity to others, and are a type of pseudo-registration. To use a normal tripcode, place a hash mark ("#") followed by a word or short phrase after what you've entered into the [Name] field (ex. "User#password"). Upon submission, the server will generate the hash unique to that particular word or phrase. The previous example would display "User !ozOtJW9BFA" after being posted.
In threads on ceirtan boards, ie /pol/, each user in the thread is assigned a tripcode. This means that when a discussion occurs, individuals can recognize which responses come from which posters. It gets rid of confusion over samefagging(someone replying to their own post) etc. Hope this helps!
Edit: Just made it this far in, but they actually tackle tripcodes in section 3.2 of the paper