Salt is a random-ish hash used as part of (factored into) a cryptographic algorithm. EG commonly passwords are first "salted", then hashed -- so that if the encryption was ever broken, reversing it would only reveal garbage without ALSO knowing the salt. Often, salts consist of simply using the time/date or something, but this still works well because in that case, you'd have to know the exact time/date/second in most cases a PW or secret was generated. They can alternately be a static character string [or anything else], or some combination of these things.  It was apparently compromised or something, but when it was changed, the tripcodes that are generated for a given input also change as a result.