The Transition Interface and it's subclasses are used to interpolate values. It's extensively used by the Tween-Class. So you can extend the type of tweens by implementing your custom Transition-Algorithm. Of course, Transitions can also be used on their own.
Have a look at the Tween-Visualizer-Example to watch all transitions that you can use out of the box.
With Tweeners you are able to create movements and animations in a very simple way. A Tween-Object can consist multiple TweenValue-Objects and a Transition-Object. The Values of the Objects are then updated over time using the assigned Trasition-Algorithm. You can used time-baed or delta-based timing.
Check out the Tweener-Example to see how easy it is to use those techniques.
A Timeline is like a TweenManager that can contain multiple tweens (playables) that can be played parallel or sequentiell. It's perfect for creating complex animations and tween sequences. As a Timeline also implements the Playable-Interface you can also create nested timelines.
Check out the Timeline-Example on how to use them.