The Listener is the "leader" of the Black Hand, the inner circle comprised by the best assassins in the Dark Brotherhood. They visit the Night Mother, who telepathically translates to them murder contracts offered by invokers of The Black Sacrament. The Listener then informs a Speaker about the contracts and an operative is sent to meet with the invoker. According to Astrid, the relationship between the Night Mother and Listener is incredibly intimate. 
Authority and power
Listeners are the sole authority of the guild, functioning only beneath the Night Mother herself. Listeners command the Dark Brotherhood and the Black Hand; they choose who to promote, who to recruit, and how. It is presumed that each branch of the Brotherhood has its own Listener to complete more contracts.
After the death of the previous listener, the Hero of Kvatch may have become the Listener of the Black Hand had they helped Arquen eradicate the traitor in their ranks. After becoming the Listener, the Hero would have taken orders from the Night Mother weekly and brought the information to Arquen, earning 200 . Brothers and sisters of the Dark Brotherhood will accompany the hero if asked.
The Dragonborn might have become the Night Mother's Listener during the period known as the Dragon Crisis, after her Keeper, Cicero, brought her remains to the Falkreath sanctuary. Although disappointed that he was not made Listener, Cicero congratulates the Dragonborn, mentioning that the Night Mother has gone without a Listener for a very long time.