Hi Native Tech.
Spine is a separate application for creating "bone-based" 2d animation. This is where you separate the parts of your character (or prop) and attach them to a skeleton where you can animate the positions. AppGameKit v2 includes support for loading and playing animations created with this tool.
http://esotericsoftware.com/
The tweening commands refer to the time-based animation of various properties of various engine components (such as sprites, objects, cameras, so on.) This allows you to automatically animate the position, say, of a sprite from one point to another over a period of time. You use just one initial set of commands to set it up before just letting it do its thing. Another benefit of tweening is the possibility of using what are referred to as "easing functions" to control the movement. So instead of just linearly moving the sprite from point A to B in x amount of time, it will instead "ease" the sprite into point B in a smooth way. You can see the tweening example included with v2 for some idea of what the different easing functions look like. Keep in mind that you can animate many different properties using these commands--you can get really creative with it and implement some cool effects (fades, movements, color-shifts, etc, etc).
(As a side note, you could theoretically implement a Spine-like animation system for your characters using the tween commands, but that's what Spine is for.)
