What does "RE" stand for at the beginning of a memo or letter?

Reference

It could mean reference or regarding, both mean the same. It is the standard
abreviation for telling the person/company who the memo or letter is addressed
to, what it is concerning. The same as the subject area when sending an e-mail. If you need a source, you can check out

http://www.macmillandictionaryblog.com/business-letter-format

There are dozens of sample business letters there. I just don't have time to
look at them all, but I'm sure you will find what you need. If not, just look
under business letter writing on the internet, and you will find numerous sites
that should be able to help you. I hope I have been of some help. Good luck.

Re is a Latin word meaning 'about'. It is a left-over from the days when many highly educated people spoke Latin and would use Latin words in business.