Adding a P.S. means that you are adding a note to the end of an email or letter, after your signature. It stands for post script, which is something added after (post) the main part of the letter (the script).
Generally, this is something that does not relate directly to the content of the letter, but is something that you want the reader to know. It is often an aside or just some extra information that did not fit the rest of the letter.
