A junior adjutant was a low-level diplomatic official who represented a government. Such officials usually were dispatched to meet or talk with those whom the government did not deem to be worth the time of a higher ranked diplomat or ambassador. They also might be dispatched if the government simply did not wish to meet with the interested party.
In 2368, Jean-Luc Picard attempted to contact Chancellor Gowron to ask the favor of the use of a Klingon Bird-of-Prey for a mission. Three days of hails failed to get a response, which Worf speculated was due to Gowron's rewriting of the history of the Klingon Civil War and his desire to not be reminded of Picard's help. Eventually, junior adjutant B'iJik contacted Picard to say Gowron was too busy to talk to him. Veiled threats by Picard, however, was able to convice B'iJik to persuade Gowron to provide such a ship. (TNG: "Unification I")