Tweens are used to interpolate the properties of instances. The Tween object itself controls the playback of the interpolation. Creating and configuring tweens is done with the TweenService/Create function. This is the only way to create tweens. Instance.new cannot be used for this particular object.
Note that while the configuration of a tween can be accessed after a tween has been created, it can not be modified. If new goals are needed for interpolation, a new tween must be created. Further information on how to create Tweens can be found on the TweenService page.
Multiple tweens can be played on the same object at the same time, but they must not be animating the same property. If two tweens attempt to modify the same property, the initial tween will be cancelled and overwritten by the most recent tween (see examples).
Although other methods exist for tweening objects, such as GuiObject/TweenSizeAndPosition, Tweens allows multiple properties to be modified and for the animation to be paused and cancelled at any point.