Re in email means what

A common abbreviation!

"Re" is short for "Regarding" or "In Response to". It's often used as a prefix in email subject lines or in the body of an email to indicate that the message is a reply to a previous email or conversation.

For example:

In general, "Re" is used to show that the email is a follow-up or a response to a previous communication, and it helps the recipient quickly understand the context of the email.