Congratulations, PAM. You are now an FO. It simply means Silver Medallion. The FO comes from an old DL designation no longer used that had something to do with Flying Orchid. If you do a search on that you'll find more of the history. The reason you could not find the abbreviation FO in your search is that the search function has to have at least three letters to work.

Quite impressive to get SM/FO in one flight. Care to share how you did it? Yes, you should get a commemorative package with your Silver Medallion card but it will take a while. In the meantime, you still should be able to get preferred seating and all the other FO benefits. Check your status online at DL.com and if it doesn't show you as Silver call customer service and get your status updated/upgraded. Even without the actual card, as long as your new status is reflected online you are good to go.