"Re" or "RE" at the start of any written communication is an abbreviation for the word "regarding" (which means "about" in this usage). It tells the reader what the topic or subject of the report or letter Is going to be. Proper use would have a colon as punctuation after "Re" and then a blank line before the report begins.
It is not used in personal communication, but often used in business and government communication. Is is usually used to begin a form of communication called a "memo" as opposed to a business letter.
To: Ms. Smith
From: Mr. Jones
Re: Your purchase from our store