Bartimaeus addresses Jesus with a form rendered 'Rabboni,' and John preserves the same title when Mary Magdalene recognizes the risen Jesus. John explicitly translates it for his readers as Teacher.
The untranslated word lets the reader hear something of the Semitic speech world behind the Greek Gospel text.
Rabboni is related to rabbi and carries the sense of 'my teacher' or 'my master.' Its precise shades of formality should not be exaggerated into a rigid hierarchy of titles, but it is clearly a respectful address.
John's decision to preserve the term and then translate it is especially useful evidence for its meaning. The same Gospel uses similar translation techniques for other Semitic names and expressions.
Recognizing the title prevents it from becoming a mysterious proper name. Both Bartimaeus and Mary address Jesus personally and respectfully as teacher or master.
In John's resurrection scene the familiar title also marks recognition: Mary's grief turns when she hears her name, turns toward Jesus, and answers in the language of discipleship she knows.