“Teacher” (NIV) = “Master” (KJV) =   didaskalos = a teacher, one who teaches the things of God. This is the same as “Rabbi” which was the usual Jewish way of respectfully addressing a religious leader.

“Lord” =  kurios =  he to whom a person or thing belongs, about which he has power of deciding; master, lord;  the possessor and disposer of a thing. It is a title of honour expressive of respect and reverence, with which servants greet their master.

Jesus claims here to be both Teacher and Lord in the fullest sense, just as he has rendered this menial, but symbolic, service of footwashing to them. This is the first time that He has accepted this highest of homage and it immediately follows His service to His disciples as a slave.