what are data add-ons?

Short for Full-Speed Data Add-Ons.  Full-Speed Data Add-Ons , like Add-Ons in general, can be added to compatible rate plans to purchase additional services. Full-Speed Data Add-Ons allow you to conveniently purchase additional Full-Speed Data that will be available each month.  Full-Speed Data Add-Ons are the best option when you know you will need more Full-Speed Data on a regular basis and there is not a suitable alternative rate plan for you.

Full-Speed Data Add-Ons are available for $10 and include an additional 1GB of Full-Speed Data.

If you seldom need additional Full-Speed Data, turning on Pay As You Go Data and funding your Flex Bucket or prepaid account balance may be a better option for you.