It stands for "passenger name record". PNR is basically fancy-speak for the records the airline has associated with your ticket. If you entered your SM# when you bought the ticket, the number should be in your PNR. You can double check by making sure your SM# is printed on your BP (boarding pass).

Oh, and welcome to FT! If you get confused by abbreviations here, go to the top of the page and click "Help" on the main FT banner (right side of the screen). A pulldown menu will appear that includes a choice for "Glossary"--it has all kinds of abbreviations explained (including PNR).