When two disciples begin following Jesus, they address him as 'Rabbi.' John then translates the word for his readers: 'which is to say, being interpreted, Master' or teacher.
The untranslated Semitic title preserves the Jewish social world in which Jesus' first disciples encountered him.
Rabbi derives from a Semitic term related to 'my great one' or 'my master' and came to function as a respectful form of address for teachers. The fully institutionalized rabbinic office and title familiar from later Jewish history should not simply be projected backward in all its later detail into the early first century.
John himself supplies the essential meaning: the disciples are addressing Jesus as teacher.
The title helps readers picture discipleship before later Christian terminology developed. People approach Jesus within familiar categories of teacher and learner.
John will progressively reveal that those categories are true but insufficient. The one addressed respectfully as Rabbi is also the Messiah, Son of God, King of Israel, Son of Man, and ultimately the incarnate Word introduced in the prologue.