Tweeners are short for what happens in the space "between" two keyframes. Before computers it was up to the artist to figure out the in-between motion and appearance, position, etc. for all the objects for the frames in between keyframes. This was back in the day when all animating was done by hand, not that long ago actually. As Mark said, you must create another keyframe further down the timeline for Carrara to know that it needs to "tween" any property. Say for instance you move the object on frame 60, Carrara will do the work of calculating all the intermediate frames in between. That way, if you scrub the timeline to frame 35, the objects' position will be calculated and the object will be drawn where it should be for that frame. The tweener is just the end users ability to decide how Carrara will make that transition between keyframes.