The jQuery website provides a href to the data types page but shows no new information once the page is loaded.
"Tween" is actually short for "in-between", and refers to the creation of successive frames of animation between key frames. In computer animation, the term is most commonly used for Flash's "shape tweening" and "motion tweening" processes, where the user can define two key frames and Flash will automatically create the in-between frames, either morphing one shape into another over a set period of time or else moving a shape or shapes from point A to point B over a set period of time. 3D animation programs also have their own method of "tweening".
As far as it relates to jQuery, I'm not entirely sure, as documentation for it is rather sparse.