Quite literally, a "tuberling" is a small "tuber"- a tuber being a vegetable or other food grown from tubes, such as potatoes, etc.

More specifically, ktuberling is a nifty program distributed in the KDE games package (usually under "toys" as "potato guy") that emulates a "Mr. Potato Head"®. Clicking on the potato itself yields the playing of a pleasing wav (usually /usr/share/apps/ktuberling/sounds/en/tuberling.wav) of a female voice uttering "tuberling".

The toy allows you to save your tuberlings as a .tuberling file to be reopened/sent to a friend or placed in the provided ktuberling museum (/usr/share/apps/ktuberling/museum).

There is also a mac kids game called "Tuberling" that is based on the same concept.