Tween - (verb) - To generate intermediate frames in an animated sequence so as to give the appearance of smooth movement (also thank you Wikipedia)
So Sherlock, can you guess what kind of tween we're going to be talking about today? If yes, then congratulations, though I wouldn't get too excited, even my cat could figure that out. If no, then I have a query for you: How did you even boot up your computer?
Okay, okay, all mockery and abuse aside, I can admit that definition was a bit cryptic. what a motion tween, or inbetween is, are frames that are generated by moving, scaling, and rotating (or other, those are just the most common) a shape to create an animation. You know that frustrating feeling you have when all you want to do is animate a car driving by, but you have the draw is frame by frame, just to get the dang thing to slide from one place to another? That's what motion tweens are here to fix, just draw your thing, make it a shape, mess around with it, and you got yourself an animation, huzzah!
So I hope this Instructable serves you well in your journey to becoming an esteemed animator. Don't get frustrated and give up, if my words don't make sense to you, I won't go cry if you watch or read other tutorials (actually, do that, it can't hurt). But enough blah, blah, blah, ONWARD TO STEP ONE!!