This is a microSD card which comes with an open version of Android installed. Insert it into your B&N tablet (Nook Color, Nook Tablet, or Nook HD/HD+), reboot the tablet, and you'll have a regular Android tablet free of B&N's chains. The tablet will have full access to Google Play and come with a lot of useful apps.
And starting today, all cards will be shipped with the latest version of Android, 4.4 Kitkat. One of these $30 to $50 cards will turn your outdated device into one of the most modern Android tablets, giving you bragging rights over a lot of Android smartphone and tablet owners.
I've tried these cards once or twice myself without much luck, but I know several people who swear by them. And even I will agree that if you have an old B&N tablet and you want to get more use out of it this is worth a try.
P.S. On a related note, N2A also expanded into offering a similar service for the Kindle Fire. For $20 they will sell you a firmware update which you can install on a Kindle Fire, Kindle Fire HD (2012), or the Kindle Fire HD 8.9 (newer models not supported).