Meaning of “re” - Learner’s Dictionary

re

preposition uk us /reɪ/ US  /riː/
EMAIL

used in the subject line of an email (= line at the top of an email that tells you the subject of the message) to show that you are replying to an email that was sent to you with the same title:

The email was titled 'Re: Our Meeting Next Week'.
LETTER

used in business letters to refer to a letter or something in a letter you have received and are replying to:

(Definition of “re preposition” from the Cambridge Learner’s Dictionary © Cambridge University Press)